Pull the spark plug and see if it rotates then.
Possible hydrolock?

First, check the oil. If oil level is okay or even close, its probably not an oil-related failure. If no oil, shoot the mower. Assuming oil is present, now look under the housing to see if the blade is bent and is stuck on the housing.
Note: always raise the side of the mower with the air filter.
Now if recently you raised the oil fill side of the mower for some reason, its possible that oil entered the combustion chamber and locked it up. Take out the spark plug and see if it pulls through. If it does, it will blow out most of the oil. At that point, if it has a pleated air filter, you'll probably find that to be oil-soaked, so replace it. Now you can put the spark plug back in and start the mower - it will smoke for several minutes as the oil burns off.
Let's say you removed the spark plug but it still would not turn, and the blade isn't hitting anything. The next thing to check is the engine brake. When you hold the engine brake bail to start and run, it releases a flywheel brake so the engine can run. A broken cable will not release the brake. Look while some else works the brake bail to see if the lever is moving under the flywheel.
Another thing I would do is to take the spark plug wire off the spark plug and ground it or secure it so it doesn't get near the spark plug. Put on a pair of work gloves. Raise the mower deck, grab the blade and see if you can force it to move in either direction. If the blade will go only in one direction but feels very 'free', then stops like its hitting something, you have a broken engine (connecting rod). If it won't budge in either direction, the piston is 'frozen' in the cylinder and the engine is history.

I think that's enough for now. Let us know what happens.