Did you try turning the engine in both directions? If it will move in the "wrong" direction (CCW, as viewed from the front) but not in the normal one, something is jammed. Could anything have fallen down one of the exhaust ports? Less likely, but by no means impossible, especially if there was no exhaust, or just a head pipe on it, is that a mouse got in, and used a cylinder with an open exhaust valve as a winter food repository. Unless the car sat outside for a long time, in very wet weather, with no manifolds on it (providing an easy path for moisture to get into the cylinders) I would be surprised if cylinder wall rust is the reason it will not turn.