There are a few possibilities. We have an old Farmall 350 with the hand crank as well as electric starter, and occasionally, the starter will lock up in the flywheel gear, and lock tight enough we can't break it loose with the hand crank. The cure is to pull it in road gear just far enough to make the engine turn a little, and it breaks loose.
There also could be something like a seeping head gasket that allowed coolant into a cylinder, and it is causing the engine to do what is called a hyrdaulic lock. The liquid on top of the piston won't compress like air would, and effectively locks the engine.
You could have also broken a valve spring, and have had a valve drop down, and wedge on the piston. This is easy to check by removing the valve cover, and looking for a broken spring, or a valve/rocker arm that doesn't look quite right. If that is all OK, I'd try to turn the engine over by towing it.
Just to be safe, I'd remove the spark plugs (to prevent engine damage) and see if you can't get the engine to turn over in high gear, and even then, just enough to get it to move a little until I could try turning it over with the hand crank to be sure nothing is wedged.
If you can get a breaker bar on a flywheel bolt, or crankshaft bolt in the front of the engine (may be hard to do with the hand crank mechanism there) try turning the engine backwards, and then forwards, to see if it is only stuck in one direction. That points to hydro lock, or catostrophic failure of something internal.
