Perkins AD3 152 Diesel Engine "stuck" and won't crank in a 65 Massey Ferguson tractor? 

Not sure what's causing this Perkin AD3 152 diesel engine to not crank here in the old 1965 Massey Ferguson 203 tractor. Could it be a stuck or siezed engine from sitting for only a few months? It ran perfectly before I parked it! WTF, is this thing for this engine? Could it be "hydro-locked?