The advancement of humanity throughout history has ebbed and flowed like the tides, there are periods where significant progress has been made and periods were knowledge of that progress has been lost.

In practice I am an art/architectural historian, and in answer to this question I can provide a specific example of this.

The Romans developed a recipe for what we recognize as concrete today, and realized over the course of building the potential uses it could have within architecture.

One of the most notable Roman buildings, still very much in tact today, is the Pantheon in Rome. The Pantheon is domed, with an oculus in the center, constructed of this specific Roman concrete.

Very much like today, the knowledge of how to construct domes like the pantheon was known and taught only by the people who were concerned with it, aka it wasn't within the public sphere of knowledge. For example - if suddenly tomorrow all the engineers and architects in the world passed away, the average person wouldn't know how to do all the things that they did without having to deeply study and understand engineering.

A presumption a lot of people have is that the fall of Rome happened over the course of one or two days, but really the fall of Rome was a general societal collapse. The Roman economy went haywire, and because most of the famous Roman architecture we recognize today was sponsored by the Roman government, there were no new buildings on that scale made.

Although the knowledge wasn't "lost" per-se, because there are a handful of examples of surviving books of Roman architects (notably an architect named Vitruvius) - it wasn't used, nor was it practical. And because these books didn't contain things like the recipe for concrete, or how these buildings were constructed in a practical sense, the knowledge was "lost."

It would be like having the parts to an object you want to build, knowing what it looks like, but lacking the user manual with the instructions.

By the time you get to the early years of the Renaissance, society has become wealthier, the government can spend money on large architectural projects, and they began to enjoy looking back and admiring the architecture of the Roman Empire. This led to a desire to try to build a dome in Florence.

The new dome would eventually be built, but over the course of a hundred of years with trial and error. Brunelleschi, the artist/architect who would complete the Dome, looked to the Pantheon for inspiration yet came up with his own solution for dome construction.

So, to answer your question, between the fall of Rome and the Renaissance, there was a "loss" of knowledge in the sense that the knowledge that was held previously couldn't be applied due to societal collapse.

It was more like remaining in an in-between space, where the knowledge was there, but it couldn't be applied. And in some cases it was lost entirely due to, again, societal collapse.

Let's say you were born within Roman walls in the year 1000. You would walk around what you knew to be ancient building, but exactly how old, nobody knew. You kept your sheep in a field with great architectural ruins, but from who? you didn't know. Your neighbor sells their goods in a large barreled auditorium space. You probably recognize that it's old, but it's just a part of your everyday routine.

So you probably wouldn't think you're in the most advanced age of history, you would recognize the advancements of those who preceded you, but you also knew that any "new" buildings built weren't on the same level of engineering and power.

The current society we are in is still going off of the Renaissance. When America was founded in the 1700s, for example, that was only ~200 years after the Italian Renaissance which in the context of history, is nothing. We are still in the upward trend, and so we feel that we are advancing with every decade. This unfortunately has not been the case throughout time.

Because of how connected we are, it's hard to imagine that there would ever be a time where it feels like we are wandering within the ruins of a once great society, where knowledge is lost or no longer applicable, and recreating it feels unattainable. But that was unfortunate reality of humanity - we can gain knowledge as easily as we can lose it.

Sources:

Mogetta, Marcello The Origins of Concrete Construction In Roman Architecture: Technology and Society In Republican Italy. Cambridge University Press, 2021.

Vitruvius Pollio, and Thomas Gordon Smith. Vitruvius On Architecture. Monacelli Press, 2003.

Ward-Perkins, Bryan, and Ebook Central - Academic Complete. The Fall of Rome: And the End of Civilization. Oxford University Press, Incorporated, 2006.

Bussagli, Marco, et al. Firenze, La Cupola Di Brunelleschi =: Brunelleschi's Dome In Florence. Scripta Maneant, 2020.

Until the modern period, the world was not exactly sharing its knowledge in a unified way, but civilisations were only chiefly aware of their own spheres of influence and regions of the world. So instead of discussing the whole world advancing technologically or scientifically and then losing that knowledge, the answers would have to be regional. The European Medieval Period is an example that has been discussed here, but a couple of others come to mind:

1. The Bronze Age Collapse.

This is an enormous topic with many mysteries attached, so I can’t go into complete detail here, but in the 11th c. BC the Minoan civilisation of Crete, the Mycenaean civilisation mostly of mainland Greece, and the Hittite Empire in Anatolia, and cities of the northern Levant like Ugarit all collapsed, with cities abandoned, monument construction ceasing, and even literacy evaporated as entire writing systems of all of these died out. This was particularly acute in the northern half of the eastern Mediterranean, but Egypt, Assyria, and Phoenician city states also saw weakening and destruction to a degree, dynasties collapsing, etc. The reasons are still hotly debated as one of history’s greatest discussion points, but include drought, volcanic activity, increased and lopsided warfare due to the spread of iron (over bronze), and attacks by what are usually translated from Egyptian records as the ‘Sea Peoples’, as well as other more complex societal developments. Most historians suspect some combination of these formed a ‘perfect storm’, but again… hotly debated. The identity of the Sea Peoples is another massively debated topic.

To give an idea of the associated speculation among historians, they are usually simplistically presented as piratical marauders who destroyed many of these urban centres, but it may have referred to a wide variety of peoples - possibly hostile migrations of Dorians to southern Greece and Philistines to the area of what would be Israel, that might be traced to this era, have been linked to these by some (not in any watertight way!), and various hints of names have been tied to particular peoples, including Cypriots, other Greeks and Indo-Europeans, etc. We don’t know very much. It’s quite possible that a chain of events started by drought and then a domino effect of chaotic migrations of many peoples, which we have seen from the Germanic migrations to many Steppes migrations, may have been at play, and may have been a cause, an effect, or both, of this broad collapse.

The collapse of the Hittites saw their cuneiform writing system vanish, with several ‘neo-Hittite’ kingdoms, like the Lydians, emerging centuries later, speaking languages related to but not descended from Hittite. Ugarit itself collapsed and also saw its peculiar writing system (a combination of cuneiform form and an Egyptian-derived, Semitic-based alphabetic system vanish), with the Ugaritic and closely related Amorite languages of the northern Levant vanishing, being replaced by Aramaic in the centuries that followed. Greece in particular entered a ‘Dark Age’: the Minoans and Mycenaeans had their place cultures destroyed, the Minoan language and its writing died out, and the Linear B of Mycenaean Greek died too. Writing was associated to scribes attached to the palaces, and as the palaces were sacked or abandoned, they vanished too. The Greek dialects that seem descended from or most closely related to Mycenaean were later to be found only in the mountainous centre of the Peloponnese and, due to a distant migration, in Cyprus, where they replaced the original ‘Eteocypriot’ culture. Most of the Pelopennese was taken over by Doric dialects such as that of Sparta, and tellingly Greek myths speak of a ‘return of the Heracleidae’.

The Greeks relearned writing around the 8th-9th century BC, from the Phoenicians, and a system of ‘hieroglyphs’ survived in eastern Anatolia, while western Anatolia eventually adopted Greek-inspired varieties. Crete was Hellenised. And eventually Greece began to build large cities and monuments again, of a very different style, with everything from the former Mycenaean age clouded in myth, most famously the works of Homer.

2. The Collapse of the Indus Valley Civilisation.

This is a similar situation in some ways, taking place rapidly around the early 2nd century BC and gradually for some centuries after that, but earlier than (1). and in fact even less is known. The Indus Valley was one of the six major (now) traditional cradles of civilisation, and developed large cities with sophisticated irrigation and sewage systems, barter tokens, metallurgy, artwork, and a writing system that we still have not deciphered. Across the middle of the second millennium, its major centres, like Mohenjo-Daro, were abandoned.

Again, the causes seem complex, are debated, may be due in great part to drought, and are roughly contemporaneous worth massive migrations that used to be more simplistically blamed.

The Indo-Aryan migration of speakers of Proto-Indo-Aryan, a language very much like Sanskrit (or, as some would loosely but reasonably define terms, the earliest form of Sanskrit), moved into NW India, and the region of the Indus Valley Civilisation (which in fact stretched more widely from what are now Afghanistan to Gujarat), over the first few centuries of the second millennium BC. Skeletal evidence of violent deaths in Mohenjo Daro was once blamed on a dramatic such wave, but the dates and locations don’t quite match. What is true is that the IVC culture abandoned its major urban centres and may have left a late vestige until around 1300 in other archaeological cultures that do not show the same amount of advancement. The population and trade seem to have become increasingly rural.

Another factor around the same time was severe drought and a shift in the course of the Indus, which cut off trade from the Middle East and made the agricultural yield probably far too low to support the civilisation at its height. The Indo-Aryan migrations may have been an effect of the resultant collapse, sweeping in (in multiple complex waves and not one single people conquering another) where there was now a relative void, and having some other technological advantages in warfare despite not being a settled civilisation with writing or great buildings of their own.

There is some cultural continuity with both peoples, but the languages of North India are almost all Indo-Aryan now. We have no idea what language(s) the IVC spoke - thought it was certainly not related to Sanskrit - and many theories are proposed. We have not decoded their writing system (a minority view is that their writing was not actually writing - I won’t get into this, but I tend to disagree). Again, knowledge of massive construction, of their sewage systems, and their system of writing, all disappeared. Vedic civilisation would slowly emerge from the more agricultural societies over the next many centuries, and writing would only re-emerge in India over a millennium later, from the influence of the Achaemenid Persians.