Small Engine Troubleshooting Problems? Ask a Mechanic Now.
Sounds as if it may be a seized engine. A few things to look for is hydro lock, clutch side preventing it from turning over or Bendix on starter not disengaging. Hydro lock, is to much fuel tapped in the cylinder not allowing the piston to move.
After all those checks, remove the sparkplug and try and turn the flywheel manually with a wrench or socket. See if you can move it in either direction. That might free it up to get it running.