Here's a story I thought about and though someone could use it to make a setting for an rp one day:
At around 2050 A.D. European physicists made a breakthrough with the first artificial black hole and swore to contain it. Although they thought they could contain the black holes they made the devices storing them were very unstable and one day one of those containers break.
Instead of collapsing the Earth into the hole, though, it was so unstable that the black hole tore the fabric of space and time, radiating the Earth and bringing a strange new planet to orbit the Earth causing tidal waves, earthquakes, and decimating all magnetic devices and thus all electricity is gone. When the Cataclysm (as it is now called) was done, the Earth was a different world, with different landmasses, climates, creatures, and a giant planet orbiting overhead.
The surviving humans created Medieval societies and tried to live regular lives amongst newer species that ancient civilizations told only stories about and feared greatly.