You have cooked the engine. Sometimes when motors overheat, the seize up. This is moving metal parts against each other, the oil gets hot and fails, the parts sort of weld themselves together. Sometimes only a little. Sometimes permanently.
It might be that only the starter was cooked. It just doesn't have enough power to turn the engine. On most engines, there is a pully bolted to the end of the crank shaft. It might be possible to put a wrench on this bolt and break the engine free. Or break the bolt, but you have little to lose. If the car is a manual transmission, it might be possible to bump it free. I did that once. Just put the car in low gear. Push the clutch in, get the care moving, let the clutch out. It should turn over, if it is ever going to.
The good news is that changing engines is not difficult. Far easier than fixing an engine.