Fall Of Empire

Poul and Karen Anderson describe the withdrawal of the Roman Empire from Northern Europe in the King of Ys Tetralogy;
Poul Anderson describes post-Roman Britain in "Time Patrol";
he also describes the post-Imperial galaxy in the Long Night and Allied Planets periods of his History of Technic Civilization:

"Now the Empire has fallen, the Long Night descended upon that tiny fraction of the galaxy which man once explored and colonized. Like Romano-Britons after the last legion had withdrawn, people out in the former marches of civilization do not even know what is happening at its former heart. They have the physical capability of going there and finding out, but are too busy surviving. They are also, all unawares, generating whole new societies of their own.
"I do not, myself, believe that history will necessarily repeat itself to that extent. Nor do I deny that it might. Nobody knows."
-Poul Anderson, Flandry's Legacy (New York, 2012), pp. 543-544.

Nobody knows what will happen by 3900, but maybe by then society will better understand its own historical processes?

We see something of the new societies being generated in the later story, "Starfog."

One planetary population has been isolated from the rest of humanity not only during the Long Night but also, before that, during the entire period of the Terran Empire. That is long enough for humanity to have evolved into something else and that is what explorers restoring interstellar civilization find on the planet Gwydion.