Pre-human Civilizations: Where did they go?

[Floridian Dream]

I've been thinking of writing a story in which humanity discovers that civilizations have come and gone long before our species first appeared, that there was sapient life as far back as the Cambrian period. What I struggle with, however, is figuring out how these civilizations could've disappeared from the face of the Earth and yet the people who inhabited them managed to evade extinction. Is extinction truly an unavoidable scenario for these pre-human intelligences, or could they have fled to elsewhere to evade such a fate? And if they did manage to escape the apocalypse, what kept them from coming back?

Could they have fled to habitable caverns or the unexplored ocean depths (this case of which I mostly see in aquatic sapients)? Were they advanced enough to seek refuge elsewhere in the cosmos, perhaps beneath the surface of the moon or Mars? Could they even have ditched their own solar system for one with a planet of more suitable conditions?

If not an extinction event (or at least some event in which the conditions of the Earth would've changed so significantly that a significant amount of life must have been wiped out, such as an Ice Age), what else could brought these civilizations down? Is there any conceivable chance that there were survivors whose descendants manage to perpetuate the species? If so, where could they possibly have gone to? Why haven't we found them by now? Why haven't they returned yet?

For example, say there was a civilization of dinosaurs that existed over 65 million years ago; if the asteroid impact hadn't wipe them from existence, where did they go to? What kept them from coming back? And if not the K-T event that drove them away, what else?

[Ariel]

Some good questions. Not sure what to suggest, but it sounds like it would be a great story!