Engine over heating coolant tank exploded

Sounds like a major coolant loss! The coolant temperature sensor is designed to work being submerged in the coolant. When you have a major coolant loss the sensor is no longer submerged in the coolant and you don't get an accurate reading on the temperature gauge. It will go back to cold. Look for a blown hose or other good sized leak. Pour some fresh coolant in the cooling system and I bet you'll see it running right out. Then you can repair the leak. Hope that helps! Jim

Bad head gasket can pressure up the reservoir causing it to blow.