The fall of Empire generally produces a dark age. We know of several in human history. One came after the collapse of the Minoan/Mycaenean imperial civilization, which fell so far that writing itself was lost—indeed, we have only recently been able to read one of their languages, Linear B, and we still cannot read two of the others. This first dark age came about after the fall of Troy and continued until well after 800 B.C.
The great Dark Ages came after the fall of Rome, and lasted for seven hundred years.
We have not yet experienced a third, and we can only speculate on how long that might last. We do know this: books are widespread. So are computers. Technical artifacts abound. No matter how complete the destruction, some will remain to be found, and used . . .