It will still be flat.
The world from space will look much the same as it does now. But down on the surface, most of the ruins of the collapse of civilization would have crumbed back to the soil and become covered with either shifting sands or dense growth.
Tribes of humans will still live an early industrial age existence with simple machines to grind grains, print books, make clothing, and even send analog messages over wireless. The global human population may number a few million people tops. In most jungles, forests and grasslands, other species will have returned to an existence similar to what they enjoyed before humans swarmed all over the planet.
There will still be books, movies, printed pictures, and maybe even a few highly prized robust computers around so people then can see how we the ancients lived. I am sure they will be appalled.