Overheating after having heads replaced

I have a 94 Lincoln continential....we just had the heads replaced with two new ones because the car overheated and cracked the heads. We have driven the car about a week and a half and it has started overheating again. It ran fine for awhile and then all of a sudden, we heard a "pop" ( sounded like a tire blew)..and the car shut off and would not start. Mechanic said a water hose came off, but after he replaced the watre hose the car was still running hot. I took it to a reputable shop for the head replacement and I assumed they checked the thermostat, water pump, and hoses. The mechaninc said he now has to replace the head gasket??? What could have caused this?? And why does it continue to overheat? thanks for any info