This is almost always a power supply issue. The thermostat needs 24VAC from a C-wire or the included adapter. Start with the basics.
- Check your home's circuit breaker for the HVAC system. Reset it if tripped.
- Verify the power switch on the furnace or heating/cooling unit itself is in the ON position.
- Make sure the furnace door or access panel is closed securely. Many systems have a safety switch that cuts power if the door is open.
- Confirm the C-wire is connected. At the thermostat, the wire in the C-terminal must be secure. At the furnace control board, the other end of that same wire must be connected to the C-terminal.
If that didn't work: If you used the C-Wire Adapter, ensure it's plugged in and installed correctly per its guide. If you connected an unused wire as a C-wire, double-check both ends are firmly seated in the C-terminals and that you didn't accidentally disconnect an existing wire at the furnace.
Heads up: Never start installation or handle wires without first turning off power at the breaker. The thermostat's own switch does not disconnect high voltage from the wires.