Have you made certain that the engine is not stuck? I’m guessing that if the spark plug is removed and it’s in gear, you can turn the rear wheels. The piston maybe stuck or the gears in the transmission are damaged.
Another guess would be that the ground wire is either broken or loose. That would cause the motor to shut down and not restart.
I’m assuming that the starter is not spinning, so the bendix is not being pushed into the flywheel. Or is the starter spinning and the bendix no engaging? At this point you might want to bench test the starter motor. But can’t see how a starter would have stalled the engine. There is an interruption of electrical power to the engine while shifting. Don’t have any idea if the interruption includes power to the starter.
If you have a voltmeter, check the on/off and start switches.
LAD Specialties customer / tony kart / rotax / kt100