2001 Prius high temp light on

My daughter has a 2001 Prius with about 165,000 miles on it. She was driving it and the red high temp light came on and stayed on. She also said that a few minutes later she lost power. She stopped and turned it off. Waited a few minutes, and started it again. The light went off and she drove it home about 5 miles. The light came on when she got home. No engine codes. I changed the thermostat. I filled it back up with fresh antifreeze and water and took out to test drive. No leaks. 2 miles down the road the light came back on. I stopped and checked it. upper radiator hose, hot and lower hose cold. Radiator not warm. Blowing out hot air with the heater. I think the water pump belt is on, as i am at work right now but will check as soon as I get home. I took the old thermostat and put it in a pan of boiling water and it opened, so apparently it was the thermostat. Anyone have any ideas what it could be. Any help would be app