DIAGNOSIS The thermostat isn't getting 24VAC power from your HVAC system's Rc wire, which is the primary power feed. This is almost always a wiring issue at the thermostat or furnace.
FIX
- Turn off power to your furnace/air handler at the breaker.
- Pull the thermostat display off its wall plate.
- Check the red Rc wire is firmly seated in its terminal on the plate. Re-seat it.
- At your furnace control board, verify the same red wire is securely connected to the R or Rc terminal.
- Restore power. The display should boot.
IF THAT DIDN'T WORK The 3V battery in the display may be dead from trying to run without power. Plug the display into a USB charger for 30 minutes, then reattach it to the powered wall plate.
HEADS UP If you have both an Rc (cooling power) and Rh (heating power) wire, the internal jumper on the Ecobee plate must be in place. Removing it when you shouldn't causes this exact error.