What should I do if my thermostat display is blank?
If the display is blank, check the circuit breaker and reset if necessary, and make sure the power switch at the heating and cooling system is on. Also verify that the furnace door is closed securely and that the C wire is properly connected. (Page 61)
From the official Honeywell RTH6580WF Wi-Fi Smart Thermostat manual, page 61
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