The first thing that comes to mind is this. The bolts that connect the engine and the Main Generator coupler plate are Missing, broken off.
Pull the injectors out, and turn the engine over with your ratchet, see if the engine will make a couple of revolutions.
Be advised, if water has leaked into a cylinder, that could be your hard stop. With the injector removed, this fluid WILL be ejected at considerable pressure. If you are standing someplace in the "line of fire", it will be unpleasant.