Make sure the thermostat is set correctly by checking it. Set the fan to "auto" and the thermostat to "heat." To see if the heat comes on, set the thermostat 3 to 5 degrees higher than the room's current temperature.
Make sure that in the rooms where you want the heat, all vents are open. Additionally, make sure the filter is clean by checking the cold-air return. Filters that are soiled accumulate dust and must be cleaned frequently to function properly. Replace the filter if you are unable to clean it.
Make sure the gas valve is turned on and supplying the fuel needed to produce heat. The valve's handle is parallel to the pipe when it is turned on. When it's not working, it's parallel to the pipe.
If the furnace is powered by gas or propane, you must reset it in accordance with the owner's manual. Normally, to complete a reset, the reset button or switch must be turned off for five minutes before being turned back on. Make sure your furnace's pilot light is lit if it has one. If not, reset the pilot light according to the manufacturer's instructions. In pilot-less furnaces, if after performing all of these steps the furnace still does not produce heat, a new igniter may be needed.