Take a ball-peen hammer or pry bar or 1/2"+ socket extension or whatever you can find that's long enough to tap on the starter at the same time someone is cranking it. It could just be bound up inside and banging on it may free it up.

-or-

If it's a manual put it in neutral, roll it forward (down a hill is easier), jump in, push in the clutch and put it in gear, then release clutch...if the engine is indeed getting fuel, air, and spark then it will start right up.