When both your wall thermostat and your heating system fail to work simultaneously, it usually indicates a complete loss of low-voltage 24-volt power to your HVAC control system. First, go to your main electrical service panel and verify that the circuit breaker dedicated to your indoor furnace or air handler hasn't tripped. Second, turn off the breaker, remove the furnace service access panel, and inspect the control board for a blown 3-amp or 5-amp automotive-style blade fuse, replacing it if the internal wire is broken. Third, check your thermostat display for dead batteries and replace them if necessary. Fourth, inspect all low-voltage wire terminal connections. You can verify that the power interruption has been resolved when the thermostat display illuminates and your heating system responds to temperature calls.
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