What problems occur when an engine overheats?

Answer 1

The excess heat makes the parts expand and "seize" effectively melting their molecules together.

Depending on the extent of the overheating, the most common failures are warped cylinder heads. If the engine overheated to the point it would no longer run, the piston rings probably shattered as well. At that point, it is usually uneconomical to repair and the entire engine should be replaced. The car stops and possible engine replacement may be required if it is an old car.