2005 Ford Escape XLT

Heater on my car quit working on Sunday, then this am on my way to work, the temp gauge went to hot. pulled over, shut off car, no steam or fluids leaking, and no hot smell. Waited a couple of hours, put about 1/2 gallon of antifreeze in car and limped it home. Temp gauge never went over half way, any idea what this is?