The answer to 'furnace not working' is that there are several common causes, including a tripped circuit breaker, clogged air filters, thermostat issues, and problems with the furnace itself. Symptoms may include the furnace not turning on, blowing cold air, or making strange noises.
For minor issues, homeowners can try resetting the circuit breaker, replacing air filters, or adjusting the thermostat. However, more complex problems like a malfunctioning control board or a gas leak require the expertise of a professional HVAC technician. It's generally best to call a pro if you're unsure of the cause or don't feel comfortable troubleshooting the issue yourself.