DIAGNOSIS The thermostat has no power. The most likely cause is an issue with the 24VAC power from your HVAC system's control board.
FIX First, check the furnace/air handler power switch and circuit breaker. Confirm they are on. Remove the thermostat from its wall plate. Using a multimeter, check for ~24VAC between the Rc and C terminals on the wall plate. No voltage here means the issue is in your HVAC system wiring or transformer.
IF THAT DIDN'T WORK If you have 24VAC at Rc and C, the internal fuse on the thermostat's circuit board may be blown. This requires opening the unit or contacting Emerson support for a replacement.
HEADS UP If your old thermostat didn't require a C wire, the Sensi Touch does. A missing or incorrectly connected C wire is the most common cause of a blank screen after installation.