Diagnosis
The thermostat has no power. This is a total power loss, distinct from a 'no power to equipment' error.
How to Fix It
Check your furnace breaker and the ON/OFF switch at the furnace itself. Ensure they are on. If power is confirmed, remove the thermostat from the wall plate and check for 24VAC between the R (red) and C (blue) terminals on the plate with a multimeter.
If That Didn't Work
If you measure 0V between R and C, the issue is at the furnace: a tripped safety, blown 3A fuse, or a faulty transformer. If you measure 24V, the thermostat unit itself has failed.
Heads Up
Never wire or measure voltage without first turning off the breaker to the furnace. 24VAC is low risk but shorting terminals can damage the control board.