There's one thing I just thought of, but highly unlikely. The starter engages with the flexplate that's bolted on to the back of the engine and inside the transmission bellhousing. If the flexplate had cracked, which is a rare but documented failure, it might be preventing the engine from turning over. I agree with nissanh and see if you can turn the engine over. Get a 19mm socket and a long 2' or 3' breaker bar and put it on the crankshaft at the base of the engine (the big pulley). Turn it clockwise and see if the engine turns with no obstruction. It'll be hard to turn as you're compressing air in the pistons but it should move.