A seized motor is usually caused by:
1) no oil, piston has stuck to the cylinder wall or (less likely) bearings have bound up or come apart and are jamming things.
2) overheated, due to lean condition, detonation, overwork, wrong fuel or not enough cooling. Piston stuck to cylinder or wrist pin failed.
3) Parts came loose, broke or failed and jammed. Wrist pin needles, piston skirt, magneto, drive gear, clutch
4) chain jammed on sprocket, chain too loose or came off tensioner
If you tried the clutch and back wheel was still jammed up it points to clutch or chain being the problem.
If the clutch works, just the motor will not turn, pull the side covers off to see if all is well.
If so, remove the head and cylinder. If the cylinder bore is messed up, you need cylinder and piston.