You might have flooded the engine. If it hydrolocks the starter is turning hard in the direction that unscrews the nut. Try pulling it through manually first...careful! A chicken stick is advised (but if you have a wooden prop a chicken stick and a flooded engine will break the prop; if composite prop, have at it).

For a chicken stick I use a length of about a 3/4 inch OD heater hose from the auto parts store and jam the right size dowel onto it. I always have a chicken stick in my flight box. Sometimes I just start the engine with it, and sometimes I use it to tell what the engine is doing without risking my fingers.

Jim