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Hello, try putting the batteries in backwards, attempt to program the thermostat that way, then replace the batteries in the proper positions. That has about a 20% chance of solving the problem.
If that fails replacing the thermostat will be necessary.
If you can give me the brand and model number of the thermostat and tell me what color wires go to which terminals on the thermostat or send me a photo of the thermostat wire connections I can advise you on how best to select a replacement.
We can go from there.