GlassJaw (thread title) said:
how does a culture recover from an apocalyptic event?
Likely, it doesn't. If the population is decimated to such an extent, societies and cultures
crumble.
Assuming that destruction was spread all over, population density drops and people band together into small groups, extended families and tribes; here and there there could be a few surviving medium-sized cities, but they struggle to maintain control over enough farmland to support the population and repel raiders: such places are likely to be dangerous places ruled with an iron fist by the military. Following this fragmentation into small communities, languages diversify, mesh and change even faster than what they would under an unified culture: after 2k years the discrepancy will be significant (think romance languages vs. latin); some old languages might be lost and forgotten, unless reconstructed through academic research.
Technology and science (in the widest sense, including magic and other esoteric practices) drop as well. As the struggle for survival becomes paramount, any knowledge that doesn't offer an immediate and practical benefit is likely to be forgotten, as well as any technology that requires large scale, coordinated efforts or a wide range of competences.
Practical knowledge that is still too technical will probably devolve into rote learning and empirical applications; depending on the general attitude, it might even take esoteric or religious overtones (if it didn't already: it's D&D after all).
As civilizations rise once again, many lost tech will eventually be recovered or developed again; depending on the setup, the new world might still struggle after 2k years to reach the former glory, or it has already surpassed its predecessor by a long shot.
Of course, science could take a different route, especially if the old one is responsible for the downfall; you may well have a new world that is superior in a given area but has lost much in another.
For example, a fallen D&D world might turn into a modern-like world: people talk through the Internet, but only a few secretive cabals still maintain actual magical knowledge.
We can aim to a more extreme development by assuming that the disaster upset the metaphysical makeup of the world; in that case, what is and isn't possible becomes relative; most old world spells and magic items might be diminished, unreliable or no longer working at all, while wizards struggle to develop new techniques that didn't work until now.
New cultures will spread to fill the gap, probably with a more fatalistic and savage bent; with a 2k years lapse, they can eventually evolve into anything. As new nations rise, struggle and fall again, the old boundaries will be relegated to history books (if they are remembered at all); in fact, pretty much everybody starts back from scratch: it is entirely possible that races and ethnic groups that were a minority in the old world rise to prominence (think about it: if civilization has just gone down the toilet, people who were already primitive raiders before the fall get a jump start).
How much the new civilizations remembers about the old world depends on how much information survived the fall; they might have a reasonably accurate (if romanticized) idea of the old world, or they might tell tales about the Earthly Heaven and how the angry gods punished some Original Sin by condemning mortals to a life of struggling.
Of course, the presence of long lived races like Elves and Dragons foils this thoughts,
provided that they spill the beans. Maybe they didn't survive; maybe they have their own shames to cover up and don't want to talk; maybe most people wouldn't believe them anyway. And for a twist: maybe they are spreading and fostering a false conception of the old world, indulging into historical revisionism to the benefit of some secret agenda (think the Immacolates in Exalted).
And now to gods: if the gods are heavily embroiled with mortals, they might act to preserve what they like through their clerics; religions will likely remain stable unless the disaster happens to be a divine war that destroyed the previous pantheon. However, if they are unfathomable and distant like in Eberron, religions may become almost unrecognizable.
If the gods instigated the fall to get rid of an unworthy civilization, they may want to start over with new guises and new names; or maybe the old gods are all but forgotten and replaced by new gods (maybe ascended mortals that reached godhood after the fall): then, if the old gods have a penchant for tentacles and non-euclidean geometry and the stars will soon be right, you'we got a problem.
Or, the previous civilization either was materialistic or worshipped false gods; after the fall, the beleaguered survivors sought and found the true gods, who gladly helped their newfound followers.