The display on my Smartstat, TH115, TH114, Easystat, PB112 is blank.
Try cycling power using the POWER button behind the left door of the thermostat.
While the thermostat is off, cycle the power at the circuit breaker powering the stat. If that doesn't work,
Remove the entire thermostat from the wall (not just the faceplate), and verify that power is getting from the breaker to the back of the thermostat.
If there is power present on the LINE wire connections, the thermostat has failed and needs to be replaced.