I landed a world with a couple of dwarf civs, one of them apparently extinct. Its sites were next door to an extremely aggressive goblin civ led by a demon, so go figure. But, two of them were ruined by forgotten beasts instead of being taken over by the goblins, and were thus available for reclaiming. This led to the following set of actions:
Reclaim Esteemeddye. Spend a year attracting migrants, exploring the ruined fort, setting up basic industries, and failing to find the supposed monster scorpion. Retire fort.
Reclaim Roughlances. Spend a couple of months getting set up in the ruins, finding and walling in the "current resident," and exploring. Retire fort.
Go to un-retire Esteemeddye, but what's this? It's listed as a ruin again! Over to legends: a few weeks after retiring, the goblins came back and conquered it. Slaughtered all the dwarves in official positions in various gruesome ways, but seem to have left the commoners alone. Also, apparently a dwarf bandit gang formed there, like immediately after retiring.
But, since they didn't make it a goblin site, it's available to reclaim, which we do with a new expedition. Almost immediately, "An ambush!" I view the revealed unit, and the name rings a bell ... it's one of the dwarves from my previous embark! I find several of these, including an outpost liaison who was apparently still on-site when I retired the fort. Although hostile, none of them react to the new dwarves presence at all. Otherwise, everything is exactly where I left it. This includes the seeds in the farm plots, which apparently went into stasis for the duration.
After a few weeks, since we evidently aren't being attacked by a horde of goblins, I retire again to check the populations in legends. My previously dead civ now has a whole passel of royalty! Seems in the weeks since I left them, the dwarves at Roughlances decided to declare themselves nobility, and have become barons, ambassadors, generals, and a queen. Guess that means I resurrected the civilization.