Why is my furnace not working and how can I fix it?

There are several reasons why a furnace may not be working, such as a faulty thermostat, clogged air filters, or a malfunctioning pilot light. To fix it, you can try checking the thermostat settings, replacing air filters, or relighting the pilot light. If these steps don't work, it's best to contact a professional HVAC technician for further diagnosis and repair.