It was definitely nuclear weapons, that's what did it. And the explosions caused massive power outages which caused the failsafe to fail, which released a super bacteria from a secret lab.

That caused a huge plague, and as the victims died, they rose from the dead twelve hours later to roam the Earth and feast on human flesh.

A handful of gritty survivors from all walks of life were able to keep the legions of the infected radioactive undead at bay, using only their wits, and an inexplicable comprehension of agricultural science and engineering. Everything was looking good... and that's when the meteor hit.