2000 Nissan Altima won't engage in any gear

I have a 2000 Nissan Altima. I was driving the other day and it done fine about a mile I stopped at the store left it running and came back out I put it in reverse and it sounded like I hit something from behind I tried to move back toward but it would not engage. The shifter moves through the gears and you can hear it changing but just won’t engage in any gear. What could have happened.

Quite a few things could have happened but there is no way for us to tell from your limited description. I suspect it is just curiosity on your part since the car has probably been towed to a mechanic who is investigating things right now.

Hopefully, it isn’t too expensive. Sorry

This does not address your mechanical problems but please stop doing that. Your vehicle could be stolen so quick and some cities actually have the ability to fine you for doing that.