Assuming you disconnected the drive chain or belt or whatever other external things could stop everything from moving...
the crankshaft bearings could be shot.. or the connecting rod bearings.. or the piston could be stuck in the cylinder for some reason.
The easiest thing to check is the piston. Take off the head and then attempt to pull the cylinder off. If the cylinder comes loose from the engine block but the piston won't allow you to remove it, the piston is stuck in there.
If the piston is not stuck and the cylinder does come off ,see if the connecting rod is free to move back and forth a bit. If so the problem may be with the crankshaft bearings.
Be advised that you will probably need a new cylinder/block gasket .. You are getting into territory that requires some experience with engines. There are many things to be careful about that a mechanic would take for granted that you do. I have no idea what tools or experience you have. Go only as far as you feel comfortable with and then ask for advice.
BTW, if you don't mind, post the type of bike and it's age and general condition.. and how and when you acquired it.