Clay tablets are actually pretty good storage, as long as the language isn’t lost. For us, though, once the lights go out, we’ll lose everything.

Our current civilization has an effective memory span of about 30 years, one generation, in both living memory and records. Our current best storage material is acid-free paper. It is far more durable than any digital medium, although nowhere near as good as parchement or baked clay. We have clay records from 5,000 years ago and parchement from over 1,000 years ago. However, anyone who has been in the stacks of a major library knows that virtually all the original paper documents from as recently as 40 years ago and earlier is crunbling to dust.

An economic/political collapse that produced a dark age would erase virtually everything. Only extreme luck and super human efforts have left us with anything from the ancient Greeks and Romans, and virtually nothing from many other civilizations, early historic India being a good example.