When residential power returns after an outage, a non-working thermostat is frequently caused by a blown low-voltage control fuse on your indoor furnace or air handler control board, which is triggered when power surges back through the system. To resolve this, first switch off the main circuit breaker to your HVAC unit for safety, remove the furnace service access panel, and locate the control board. Inspect the small 3-amp or 5-amp automotive-style blade fuse; if the internal wire is broken, replace it with an identical amperage fuse. If the fuse is intact, your thermostat may simply need a hard reboot by removing the faceplate, taking out the batteries for a minute, and reinserting them. You can verify that your troubleshooting was successful when the thermostat display screen powers back on and resumes normal communication with your heating and cooling equipment.
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