Bud,

Yes could be hydro locked as Rcer said. Sometimes if you didn;t tighten the fan/clutch assembly to the crankshaft enough , when it hydrolocks it will loosen and jam everything up into the clutch housing.. If it won;t crank with a heavy starter, even when the plug is loose / out , your fan has probably loosened and jammed up into the bell housing.

Hope you get it sorted.