We can only hope that said asteroid isn't too big and evacuate the area of impact. We can't do anything to prevent it. ICBMs can't even target something in space, and even if we blow the asteroid with one the same mass would still be flying towards earth as a shrapnel. Theoretically we can try shooting it with a laser to evaporate part of it creating thrust to divert it, but we don't have lasers that powerful and probably wouldn't. This is uncomfortable truth - we are completely defenseless against asteroids.