Can you turn the flywheel? If you can turn the flywheel, then the engine's not frozen and prob is with the PS itself, somethings broken. Time to pul it off and see whats going on - may be as simple as a the oneway brg. If you can't turn the flywheel, then its time to pull motor and figure out why its frozen.

If engine is fairly new and has good pinch and compression, you may need to use a screwdriver to see if the flywheel will move.