I assume that your starter motor is turning. Could be the solenoid or stripped gears on the starter pinion, but I don't see why shutting down in gear would cause either problem.
As I mentioned, you can check for out of gear by hand turning the prop shaft. You can check that the engine itself is not frozen somewhere by removing the spark plugs and trying to hand turn it. You may need to use a wrench on a coupling bolt or some such arrangement to get enough leverage (or use the hand crank on the flywheel if you have one).
If you can narrow down the problem, a possible source of help is Don Moyer's "Guest Log" at www.moyermarine.com. He will answer most questions and usually has some good input if the question gives him enough information.