Ran out of gas would be the obvious answer.
Wasn't it after hurricane Ike that a bunch of states ran out of gas?
All it takes is for the refineries to shut down, and then the trucks to stop moving it.
Couple that with high demand of massive evacuations of cities. Anyone who's not affected is running.
As far as guns and ammo, yeah, it should be everywhere.
Plot may not be bedrock solid, but its a whole hell of a lot better than the movie I watched last night. Avatar.