can someone tell me whats wrong with my zip please?

Basically i was driving my 50cc 2 stroke motorcycle which was driving perfectly fine and then as i was driving something suddenly happend and the bike would rev and make a really loud noise but not move. so i pushed it home and waited to the next day and tried to start it.
the bike actually starts but when i rev it it jus doesnt move and makes a really bad rev sound. it sound as if its going to move when on the stand but when u take it off it will rev but not move ( when i rev it sounds a little different than it should sound) i really need to get it fixed so it would much appreciated if someone helped me out to know whats wrong with it thanks.