Diagnosis
The thermostat has no power. This is almost always a wiring or furnace connection issue, not the unit itself.
How to Fix It
Check the C-wire. The Sensi Lite requires a constant 24V common wire for power. Turn off power to your furnace at the breaker. Pull the thermostat off its wall plate. Verify a wire is connected to the C terminal on both the wall plate and the furnace control board. Ensure it is secured under the terminal screw.
If That Didn't Work
Check the furnace door safety switch. Some furnaces have a switch that kills power if the access panel is off. Make sure the furnace panel is properly seated. Also, confirm the furnace breaker and any emergency switch are on.
Heads Up
If you have an old system with only two wires (R and W), the Sensi Lite will not work. It needs the C-wire. You will need to install a new wire bundle or use an external plug-in adapter.