The answer to 'furnace not heating' is that there are several common causes, including a malfunctioning thermostat, clogged air filters, issues with the pilot light or ignition system, and problems with the heating elements or blower motor. Symptoms may include the furnace not turning on at all, blowing cold air, or making strange noises.
Some basic troubleshooting steps you can try yourself include checking the thermostat settings, replacing the air filter, and relighting the pilot light. However, for more complex issues like problems with the electrical components or the need for repairs, it's best to call in a professional HVAC technician who can properly diagnose and fix the problem.