Diagnosis
The thermostat has no power. The most likely cause is a tripped system fuse or a blown 24VAC transformer due to a short in the low-voltage wiring.
How to Fix It
First, check the HVAC system's power. Go to your main electrical panel. Find the breaker for your furnace or air handler. Ensure it is ON. Next, locate the furnace/air handler itself. Open the service panel. Find the low-voltage fuse, typically a 3-amp or 5-amp automotive-style blade fuse. Remove and inspect it. If the fuse is blown, replace it with an identical fuse. Re-mount the thermostat base. If the screen lights up, the short was likely momentary.
If That Didn't Work
The 24VAC transformer inside the HVAC unit may have failed or been tripped. At the furnace, locate the transformer (usually a small, square or rectangular component with wire terminals). Use a multimeter to check for ~24VAC between the R (power) and C (common) terminals. If you read 0VAC, the transformer needs replacement by a technician.
Heads Up
A blown fuse often indicates a wiring short. Before replacing the fuse, ensure no thermostat wires are touching each other or the metal wall plate. A recurring blown fuse points to a persistent short that requires professional diagnosis.