DIAGNOSIS: The thermostat has no power. The most likely cause is a drained or faulty C-wire connection.
FIX: Check your HVAC system's control board. Confirm the blue C-wire is securely connected at both the thermostat baseplate and the board. If you have a C-wire adapter (common with Sensi), verify its power LED is lit and its connections are secure.
IF THAT DIDN'T WORK: The unit may have entered a protective lockout from a power surge. Turn off the HVAC system's breaker for 60 seconds. Then restore power and wait 5 minutes for the thermostat to reboot.
HEADS UP: If your system was installed without a C-wire and you're using batteries, blank screens indicate dead batteries. The Sensi ST55 does not have a battery compartment; it requires constant C-wire power.