Apocalypse cleanup gone wrong
First, there is a thermonuclear war that destroys most of the planet and poisons the rest. The remaining scientists come up with a plan:
They create a nanotech swarm to clean up all the pollution and robots to restore the ecosystem from seed banks. That's going to take several hundred years or more, so the remnants of humanity will wait that out in hibernation, using tech from space travel.
Fast forward some 200 years and a large group of humans is rudely awoken by the failure of their hibernation system. It's being broken down by the nannite swarm. The pods housing the scientists failed in a more dramatic way: they were flooded when the nannites broke down the water pumps housed on the same level, drowning them all the moment they were defrosted.
The humans make their way to the exit, to find that the great cleanup has gone for beyond its intended scope. The nannites have broken down all ferrous metals and most plastics in order to fertilize the land and especially the oceans. The deeply buried bunkers were among the last sources of metal, but everything on the surface is gone. Even the robots that replanted parts of the land are gone, recycled by the nannites.
There simply isn't anything left but copper and tin to work with and even when a few of the smarter survivors try to mine for iron, the nannites detect it in short order and take it away.
With all that knowledge now useless, it is not passed on to the next generations, whose lives are taken up by the struggle obtain enough food.
Soon, only stories remain about beings that resided in the heavens, could create whatever they wanted, fly, cure diseases and even revive the dead.