The problem here is that "did loss of [x library or other storehouse of information] lead to loss of information" is, by itself, a premise that doesn't work. Single, catastrophic events may be dramatic to read about after the fact, but they are not the cause of losing texts or knowledge. I commend to your attention u/XenophonTheAthenian's post on that bookshop on the Nile Delta, which is the most frequent subject of such queries; Xenophon addresses not merely the loss of the Alexandrine institution but also how we do lose texts.
The Library of Alexandria has often been considered the largest repository of knowledge in the ancient world, and the model for all contemporary libraries afterwards. Libraries had existed beforehand in places like the Near East (the Library of Assurbanipal, for instance), but the library built by the Ptolemaic dynasty was on a whole other scale.
From what we can tell, ancient authors suggest that the number of scrolls within the Library ranged from 200,000 to 700,000 scrolls (Josephus, Jewish Antiquities, 12.2.1; Aulus Gellius, Attic Nights, 7.17.3; Ammianus Marcellinus, The Histories, 22.16.13–14). To clarify, scrolls are not 1:1 with books in our modern sense: each roll of papyrus may count as “1 book”, so even though we consider something like Homer’s Iliad as 1 complete work, it would come out as 24 scrolls of papyri (1 book per 1 scroll).
Regardless, there was a huge amount of unique works stored in the Library. One of the Head Librarians, Callimachus (the famed poet), wrote a catalogue of all the works that the library possessed in the mid 3rd century BC - though the catalogue does not survive, we are told it measured an astonishing 120 scrolls in length (Armstrong, R.H. “A Wound, not a World: Textual Survival and Transmission” in “A Companion to Greek Literature”)
These were acquired thanks to the seemingly endless financial backing by the Ptolemies, who made it a point of pride and personal pet project. Some of their collecting habits are pretty legendary: according to Galen, during the time of Ptolemy II Philadelphus every ship that was pulled into the harbors of Alexandria, the ship was immediately searched and had its writings seized (Galen, 17a.606). The Library would keep the original, and return the copies to the previous owner. Ptolemy II also duped the Athenians by “borrowing” the original manuscripts for Euripides, Aeschylus and Sophocles, placing a princely deposit of 15 silver talents as insurance. He then sent new copies and kept the manuscripts (Galen, 17a.607).
Greek writings seemed to have been the focus, but there is evidence of copying and keeping non-Greek works as well. It is not implausible to think that records taken from Babylonian and Egyptian scholars and astronomers were collected, and we know that the writings of Zoroaster were translated into Greek and subject to a commentary by an Alexandrian scholar (Syncellus, Chronographia, I.516.3-10; Pliny, Natural History, 30.4). Jewish tradition maintained that the Torah (the first five books of the Bible) was translated into Greek under the direction of Ptolemy II to be kept in the Library (Aristeas, Letter to Philocrates, 26-47; Josephus, Jewish Antiquities, 12.2.1-15).
The Library has since been lost, the exact reason why is highly speculative and inflammatory (no pun intended), so I need not go any further in that regard. When it comes to assessing the consequences of the loss, there are a few camps. Optimists argue that the Library purposefully cultivated a “canon”, taking the best of each playwright/poet/intellectual and disseminating those works into the wider Greek-speaking world. This is why we have a few really good plays of Sophocles or Aeschylus, despite each writing dozens. Others are less optimistic, and the loss of any work is a great tragedy. Now I don’t think there was a scientific treatise lost in the blazes that would have sent us to the moon by the 15th century, but it really reminds us of how little we actually have from the ancient world, and how ultimately it is the fact that we will never know which is the most painful.
I actually did a recent episode of my podcast on Hellenistic Alexandria, and my transcript contains quite a few references and books to look into if you’re interested in the Library (https://hellenisticagepodcast.files.wordpress.com/2021/05/060-ptolemaic-egypt-a-travelers-guide-to-alexandria-transcript.pdf)