I accidentally hit the drawer with my chair while it was open and it wouldn't go back in, but I managed to get it off without anything breaking. How do I get it back on without anything breaking? Thanks!

If it came off, it's already broken. I really doubt if you can put it back in with any success, it has a stop somewhere that keeps the drive from spitting it out entirely, once you force it past that stop I don't think it can be successfully replaced. If you do get it back in, it has gears that open and close it, they have to mesh with a track on the drawer, that's not easy even if you manage to take it apart without breaking it. (I tried once just to see, never got it back together and working again) Problem is it has keepers and stops all over the place and without completely removing them, the only way to get the gears to mesh is by forcing the motor out of position. If the plastic doesn't flex enough, the motor shaft just got bent. Most of the keepers are not removable, either molded, melted or glued on. Same for most internal parts actually, the drive is not made to be serviceable...

A new one shouldn't cost much, I think they are around $35 or so if you shop around a bit.