Mustang lx 1984 v6

I have a mustang that died on the freeway and untill the next mourning was still turning over and would start up but run loudly and rough. When I had it towed home it still started but the next mourning I tried to start it and the crankshaft would not turn even with a torque bar hooked on the pulley and me leveraged against it from under the car. No matter how hard I push it seemed to be seized up. I need to know how this could be when it had no problem starting up the night before. I had turned the car off that night: it didn't die. Also all of the fluids were good when I checked them on the side of the freeway. Anybody out there give me some ensight into this?

 

if its a manual transmission was the car in nuetral when trying to turn the engine over by hand?
if the engine is seized up would start by pulling all the plugs then try turning it over again as it could be liquid locked due to antifreeze in a cylinder.

 

Sounds like you may have a bad head gasket leaking coolant into one of the cylinders causing hydra-lock which can seem like a seized engine.

Pull each spark plug one at a time and try to crank the engine after each one.
It only takes a small amount of coolant to cause this and probably leaked into the cylinder overnite.

Keep us posted.