If the thermostat is battery powered, check/replace your batteries. If the device is common wired, try and power it up with batteries. If that fails to power up the thermostat, you may need to involve a technician. If batteries do power up your common wired thermostat, check your breakers, furnace power switch, furnace door kill switch to insure your system is powered. If your system is fine, and the thermostat only works on batteries, you may need to involve a technician.