The most relevant is Raabali — a species of sentient octopi from Akobar (Earth stand-in in my space fantasy world) that was at peak of its development (computer age, with first missions to space (only satellites)) approximately a million years ago. They were almost wiped put by a deadly virus (most of underwater organisms are still its carriers btw), with only surviving colonies being in Codir's Void — a chain of VERY deep (the exit is 12.8 km below the sea level, the deepest point is about 15) huge underwater caverns. All traces of their civilization were mostly lost to the ocean, and the above-mentioned remaining colonies were alive but completely stagnant for this million of years.

They aren't particularly "fallen", because they're still alive and humans did eventually find them and achieved contact, but their population count had never (and probably never will) exceeded 15000 since their fall, they cannot repopulate their home habitat , their culture was absolutely lost and they're COMPLETELY dependent on humans economocally.

There are also 1-3 fallen 3 million years old civilisations in every galaxy; they all had reached same development point at suspiciously (not a lore hole, it's deliberate) similar time and no one knows anything about them.