If you have a gas furnace, it probably uses a pilot light to ignite the gas burner. If your furnace isn’t heating or won’t come on, you should check to make sure the pilot light is burning.
If you’re lucky, performing your DIY heater repair will be a simple matter of relighting the pilot light. There should be instructions on your furnace to help you. Most newer units are going to have an electric igniter for the pilot, which makes relighting it super easy.
To relight your pilot light, turn the gas valve off and leave it that way for five minutes in order for any residual gas in the air to disperse. When you’re ready to proceed, turn the gas valve to the pilot position. There should be a red button that you can press to prompt the flow of gas from the pilot burner. Press this button and hold it down for 30 to 60 seconds, while using a long match or barbecue lighter to light the gas coming out of the pilot burner. Keep holding the button down for the full 30 to 60 seconds after the pilot light is burning again, so that the thermocouple can warm up and start generating voltage.
If your pilot light won’t stay lit, you can try adjusting the flame adjustment screw, if your pilot light has one. Follow the manufacturer’s instructions for making this adjustment to your unit. If nothing you try gets the pilot light to stay lit, you need to call a professional -- you probably need a new thermocouple.