Gas furnaces sometimes have problems starting. "Furnace not turning on" is a common problem, believe it or not. There are a few things you can try to fix the issue before calling your local HVAC contractor. First, make sure that the thermostat settings are correct. For instance, make sure that the "Cool - Heat" switch is in the "Heat" position. Second, make sure that the fan switch is in the "On" or "Auto" position. Third, if you are using a digital thermostat, make sure the batteries are good. Replace them if the battery indicator is showing the battery level is low. Finally, replace your air filter. If the air filter is dirty and the furnace overheats, the limit switch may trip. Some furnaces allow the limit switch to reset a few times within an hour before it shuts the furnace down until a qualified HVAC contractor resets it.
If any of these suggestions did not work for you, it is time to call an HVAC contractor.
If you have any additional questions about gas furnaces, contact your local HVAC contractor.