The flywheel key might have sheared on the engine. Something internally might have broken inside the engine.
To check the flywheel key, remove the cover on top of the engine (blower housing). The blower housing covers the entire upper portion of the engine. It is held on by 4-5 small bolts around the lower edges. Remove the bolts and lift the blower housing off the engine.
Remove the cup on top of the engine the starter engages to and look at the hole in the center of the flywheel. There is a groove in the engine shaft and a corresponding groove in the flywheel. If these 2 grooves are not perfectly aligned, the key is sheared.
You have to remove the flywheel to replace the key. You will need a special tool to remove the flywheel. The part number of the flywheel puller is 42-074.