Yep, broken heater, lights or pump. Do you have any internal pumps that have not been cleaned in a while?
If you have MH
lights going you probably want to have a fan blowing across the top or the sump to help with evaporative cooling.
I had that problem when I was out of the country and friend was stopping by to feed the fish. The house heated up into the 90's from the lights (it was a warm November) and I had shut all the windows. I lost all my sps
and some lps
corals. I had to frag my leathers to save what was still good.