The answer to 'why is my furnace not turning on' is that there could be several common causes, such as a tripped circuit breaker, a clogged air filter, or issues with the thermostat or ignition system. The symptoms may include the furnace not starting at all, or the blower fan not running.
Some potential solutions you can try yourself include checking the circuit breaker, replacing the air filter, and ensuring the thermostat is set correctly. However, if the issue persists or you're uncomfortable troubleshooting the furnace, it's best to call a professional HVAC technician to diagnose and repair the problem.