The answer to 'furnace not working' is that there are several common causes, including a thermostat issue, clogged air filters, pilot light problems, or a malfunctioning ignitor. Symptoms can include the furnace not turning on at all, short cycling, or blowing cold air. Solutions may involve checking the thermostat settings, replacing air filters, relighting the pilot light, or troubleshooting the ignitor.
In many cases, homeowners can troubleshoot and fix minor furnace issues themselves. However, for more complex problems or anything involving the furnace's electrical or gas components, it's best to call in a professional HVAC technician. Attempting repairs on your own can be dangerous and may even void the manufacturer's warranty.