Now the Moon is pretty big. In fact, the Moon is 3,476km in diameter, that's around the same distance as travelling from New York to Beijing.
With that in mind, what are the odds of the Moon being knocked into the Earth?
For starters you don't want an object that's either too small or too big as you'll either just give the Moon a significant dent, or demolish it entirely. Neither resulting in a giant Moon-based apocalypse.
No, instead you're going to need to play some intergalactic snooker, using an object that is almost exactly the same size as the Moon, travelling at exactly the right size and at the right angle.
So as with most things in the universe, it's not impossible, just really really really improbable.