A furnace can fail to ignite for several reasons depending on the symptoms, manufacturer and model. Common problems with furnaces are restricted or blocked air flow, problems with the ignition source and electric problems with the control panel or thermostat. Ensure the furnace has power and the door is shut.
Some sensors and safety devices prevent a furnace from igniting or staying lit. A furnace fails to ignite when it has a blocked or restricted air flow. The furnace needs plenty of air flow to maintain the pilot light and to accommodate the initial spark of the ignitor. The main burners failing to come on could be a sign the area lacks sufficient oxygen. Check the exhaust pipe for sediment and build-up. Open all the restrictors and inspect the filter.
If the pilot light is out, clean and inspect the pilot assembly. Replace the thermocouple, if no pilot is present. If the furnace has an electric ignition source, a clicking should be heard as the furnace comes on. A bright glow can sometimes accompany this as the ignitor is heated.
An electrical problem at the control panel or thermostat can cause the furnace to fail to ignite. Check the wiring to and from the thermostat for breaks. Check and clean the flame sensor.