If the thermostat uses batteries, replace them or make sure that the existing batteries are still in good working order.  If the thermostat does not use batteries, verify that the furnace is plugged in and is getting power (plug something else into the outlet, like a hair dryer, and make sure it turns on). Make sure that the breaker for the FAU (furnaces) in the electrical panel is turned on. If you recently replaced a filter inside the furnace, make sure that the furnace door was reinstalled properly.  Call us for more information if the above instructions do not resolve the issue.