You are probly experiencing hydro lock. Take the glow plug out and try it then. If it is hydro lock then it will push all the fuel out the glow plug hole so watch your eyes as the fuel will spray up at you. If it is not then you may have gotten a pebble or something in it and it is stuck between the piston and on of the ports. Or the wrist pin c lip may hace backed out and the wrist pin may have slide out and may be catching one of the ports. Try the glow plug things then if thats not it you may have to tear the engine down to find the problem.