DIAGNOSIS The thermostat has no power. The C-wire (common wire) is either not connected at the thermostat or at the furnace control board.
FIX Turn off power to your furnace at the breaker. Remove the thermostat from its wall plate. Check the bundle of low-voltage wires. Look for a wire in the terminal labeled C. If there is no wire in C, you need to find one at the furnace. At the furnace control board, identify an unused wire in the thermostat cable bundle. Connect it to the C terminal on the board. Then connect that same wire to the C terminal on the thermostat wall plate. Restore power.
IF THAT DIDN'T WORK The transformer in your furnace or air handler may be faulty. Use a multimeter to check for 24V AC between the R and C terminals at the thermostat wall plate with power restored. No voltage means a problem at the HVAC system.
HEADS UP Always kill power at the breaker before touching furnace wiring. Shorting the R wire to another terminal can blow the low-voltage fuse on the control board.