DIAGNOSIS Corrupted firmware from an improper shutdown during the power interruption.
FIX Perform a hard reset. Turn off the HVAC breaker. Remove the thermostat from the wall plate. Wait 60 seconds. Reconnect it to the plate, then turn the breaker back on. It will attempt to reinstall the firmware.
IF THAT DIDN'T WORK The firmware is bricked. You need to initiate a recovery mode. With power on, press and hold the Menu and Fan buttons together for 15 seconds. The screen may flash. This forces a firmware re-download from the server.
HEADS UP The recovery process requires a stable Wi-Fi connection. It can take up to 20 minutes. Do not interrupt power during this time.