I could be a million different things from simple to catastrophic...... If you give us a little more info from when, how and what year...
But I'll do a shot in the dark... I'm assuming even with the bike off you can not get the bike into neutral, and it is stuck in gear...... Start from the shift linkage.... Check clutch cable and adjustment....... Then pull the primary and smell the oil then check the shift shaft..... Check your clutch..... If you have not found a simple problem by then then you could have a bent shift fork or bad trans or even worse..... A broken case