I couldn't find another thread that explains this, which surprised me, but here is another discussion of the Old Kingdom by u/mp96 which gives some background such that I can skip over some basics: https://www.reddit.com/r/AskHistorians/comments/27r5c8/how_do_we_know_that_the_egyptian_old_kingdom/

To begin with, know that the ending/changing of 'Kingdoms' isn't anything noted by contemporaries, but is a modern (Antiquarian) way of organizing the staggering history of pharaonic Egypt. Even then, the Dynasties which have made up the Old Kingdom [OK] and the successive First Intermediate Period [FIP] have changed (The 7th and 8th Dynasties have rejoined the Old Kingdom after previously making up the beginning of the intermediate period).

But as for the Old Kingdom, it's good to think about it as being the first time power along the Nile is consolidated into a small group of people (a Pharaoh and his immediate courtiers). The Old Kingdom 'ends' when political power in that region begins to swing in the other direction: more diffused, local power held by regional nobles. This is marked by increasing social mobility (in other words a middle class that begins to do very well for themselves), a period of successful and productive marriages between nobles, uninterrupted passing on of that power to coming generations, and eventually, the intermarrying of Pharaonic princess to those same provincial nobles as they reached equal footing in terms of political power.

As far as the archaeology of this transition, you can look at the changing trends in noble's tombs. Where at the beginning of the Old Kingdom, wealthy folks wanted to be buried in proximity to Pharaohs (for an eternity of ass-kissing presumably), by the end of the Old Kingdom nobles were being buried wherever they please and decorated their tombs with iconography typically reserved for royalty.

Another faction which dramatically increased its political power during this time were priests, who (I kid you not) lobbied for and extensively benefited from tax exemptions. Their accumulated wealth and increasing independence from royals took a good chunk of power from an already suffering centralized power.

A third and final factorthat is worth noting is (likely) a change in climate around this time. Poor flood seasons would have caused a food shortage, which at the same time would have drawn periphery populations closer into the Nile, putting further strains on resources. Think of the environment around a gas station when an oil shortage is announced and everyone converges on the remaining resources. And the station management is locked in the bathroom by the attendants. That gas station is Egypt around 2160 BCE.

And it's worth noting along with that last fact that, religiously, it is one of the Pharaoh's chief responsibilities to maintain the physical 'health' of the land of Egypt. Good kings caused good flood years, that sort of thing. A weakened system of central power, faced with two competing sources of power (the middle class and the priesthood), and symbolically undercut by a failure to fulfill their primary role, lead to the wishy-washy Egypt of the First Intermediate Period.

Egypt was by no means 'weaker' during the Intermediate period, art, architecture, and trade all flourished under local leaders, but it's less 'fun' than a nice, orderly Egypt with a single Pharaoh at the head of everything. Eventually, we enter the Middle Kingdom about 100 years later when Egypt is reunified under a strong central monarchy. Driving home the point of change between the OK and the FIP.

Hope that helps!

All information comes from Ikram 2010

(Edited twice for format and to credit user u/mp96)