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Hello, it is likely that the 24 volt transformer in the furnace or air handler that supplies power to the thermostat is failed or has blown a fuse in that circuit.
Tell me if you have or want to get a multi meter to check that voltage, if you do check the red wire on the thermostat terminal strip inside the blower service compartment to ground (any bare sheet metal on the furnace will work for that)
we can go from there
Hello again, there are a number of devices in the system that can shut off power to the thermostat, one of them that you mention is the drain pan float (optional) that can be stuck and killing power to the thermostat.
Another is the door switch on the blower service access panel can not be making good contact.
I could go on,
However what works best is a systematic approach to detecting the 24volt control voltage, that requires a multi meter, then in short order we can discover what the problem is.
First step is determining if you have 24v power to the red wire on the thermostat or not, if you have power there, then the problem is with the thermostat. If you do not have power there, then we need to trace the break in the circuit.
Trial and error works, but can take hours or days. Diagnosis can be done in seconds if you want to follow my lead.
tell me which way you want to go on this. and a lot about your technical skill sets so that I can decide which approach will be most effective in your case