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Start by checking the fuses/breakers for that circuit and it may be labeled "heater" or "furnace" in your load center. The other thing to try is taking the cover off and see if the internal fuse is good or not. If both of those look good then try unplugging the thermostat and let it sit for 5 minutes and then plug it back in. The connections may have gotten damp from high humidity. Get back to me with the results and we can go from there. If we need to do some testing a small digital multimeter like you can find at most hardware stores is very helpful here. All follow ups are included in our fee. Please include the part number off the thermostat when you reply. They have more than a dozen variations of these. It is on the back of the thermostat.
Just checking back in to see if you were able to resolve this issue or do you still need more input from me here?