Yeah, I'm not sure quite what I want the disaster to be.

Some thoughts:

Plague
Nothing like a good plague to cause chaos to a population. Feels a little cliched after the awesome persistence of the Resident Evil series, it's like the thing is a zombie or a vampire or something undead...

Alien Invasion
Again, staple of so many action movies and nearly B-grade Sci Fi stories.

Humans are too smart for their own safety
Mad Scientist A creates a device that does [insert words with 9 syllables that mean footwear, colliding matter or a state of cheese] and accidentally screws the world into a [insert vaguely plausible scenario, add Jeff Goldblum and it's almost believable... like Santa]
This includes the "Post Nuclear Winter" Scenario, the Global Warming killed us all and Some Jerk Built A Roach Bomb That Created Sentient Rats.

Meteorite
"Mummy, I wanna launch Bruce Willis into a meteorite! Does Aerosmith play when every meteorite is about to blow up the world? Why are they trying to make babies when they won't be alive for her to get the baby?"

Again, staple concept. Leads to variations on the "Chernobyl Nuclear Winter".

......

I think I have it... *puts it under wraps*
"I have a cunning plan... First we'll build a large wooden badger..."