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Hi I'm Brian and I'm here to help!
Typically when you have this situation, the 3 amp automotive style fuse on the control board has blown. It should be a purple fuse near the low voltage terminal strip and the white connector plugs on the board. There's also the possibility that there is a field installed safety switch that is breaking the voltage to the thermostat. The most common one being a drain overflow switch but usually these are wired just to stop the cooling from working, not to interrupt all voltage to the thermostat. I would check for both issues. The fuse, if blown, can be found at any auto parts store. It's an ATC3 fuse.
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