When your furnace fails to ignite, the first place to look is the pilot light. The most common reason for this problem is because the pilot light is not on. Unlike older pilot lights, which have to be lighted by hand, automatic pilot lights, now commonly found in most home furnaces, are regulated by a thermostat and only ignite when needed. While older pilots burned constantly, wasting gas, automatic pilot lights come on long enough to ignite the gas supply and then shut off. Since automatic pilot lights cannot be lit by hand, the procedure for lighting them is entirely different. In fact, it is really not a matter of "relighting" the pilot, but rather of resetting the pilot light. To do this, you must shut off the gas supply, effectively resetting the system and resetting the pilot. You won't need any tools to reset the pilot.
Set the thermostat on your furnace to its lowest setting. Shut off all electrical power to the furnace, either by using the OFF switch on the furnace or by flipping off the circuit breaker in your house.
Remove the access panel covering the gas control knob. Typically this is found toward the bottom of your furnace.
Turn the gas control knob or knobs to the ON position. Replace the access panel.
Turn the electrical power to the furnace back on. Set your thermostat to your desired temperature. After a few seconds, the pilot will light up, igniting the furnace's burners.