Greg may be correct with the stuck ring theory. Try heating the main cylinder casing with your heat gun and adding after run oil while moving the piston up and down. Heat it up good and hope enough heat transfers to the piston and ring to free it. If it seems to free it at all, stick the glo plug back in and try firing up the engine. If it starts, run a tank of fuel through it then use after run oil when your done. Put the engine in service to keep it free, and use after run oil after each flying session.