Diagnosis
The thermostat has no power. This is either a wiring issue or a failed internal display module.
How to Fix It
Perform a complete power reset. At the furnace, turn off the power switch or breaker. Remove the T10 Pro display from its base. Wait 60 seconds. Restore power at the furnace. Wait 5 minutes for the system to stabilize. Reattach the display to the base.
If That Didn't Work
Check for 24VAC between the R and C terminals on the thermostat base with a multimeter. No voltage indicates a blown fuse on the furnace control board, a disconnected R wire, or a faulty transformer.
Heads Up
If you have a heat pump system, incorrectly connecting the O/B wire to the conventional O/B terminal instead of the heat pump-specific terminal can cause a short and blow the system's 3A or 5A fuse.