I have a mountain bike and it was working well.then I started to pedal and the back wheel isn't moving.could someone tell me what it could be?
One possibility is that the hub's freewheel mechanism has broken. Maybe it's fixable with some spare parts or you may need a new freewheel hub body or a new hub altogether. Your LBS will be able to assess the situation.
It looks like this: https://www.google.gr/search?q=mtb+freewheel+hub+body&tbm=isch