The result is generally that you have the remnants of humanity fighting to survive in a hellish and devastated Crapsack World full of Scenery Gorn, Ghost Cities and a lot of nastiness in every corner of whatever remained of past civilization. Be it nuclear war (which was once very popular but has gone out of vogue, in part due to The Great Politics Mess-Up), The Plague (which currently seems to be the most popular), natural disaster (which seems the most likely to happen in the near future in Real Life), supernatural disaster (usually the case with a Sealed Evil in a Can or missing Cosmic Keystone), devastating environmental changes (which, in this kind of fiction, happen too quickly for civilisation to adapt), a Zombie Apocalypse (also hugely popular these days), or an Alien Invasion (God help us if that one happens) - most of humanity is gone.
Something hugely nasty has happened to humanity.
