The engine might be hydro-locked. A bad head gasket may have allowed coolant to fill up a cylinder. Coolant is not compressible, so the engine won’t turn. I suppose it is possible that a fuel injector allowed raw gasoline to fill a cylinder and raw gasoline also cannot be compressed. If the engine won’t turn with a breaker bar with the socket on the flywheel, you might try removing the spark plugs and try to turn the engine. If it is then free, you have the hydro-lock condition.