The answer to 'furnace not turning on' is that there are several common causes, including a tripped circuit breaker, a faulty thermostat, or a problem with the furnace itself. Symptoms may include the furnace not starting at all, or the blower fan running but the burners not igniting.
If the furnace is not turning on, the first thing to check is the circuit breaker. Make sure it has not been tripped and reset it if necessary. You should also check the thermostat to ensure it is set to 'heat' and the temperature is set higher than the current room temperature. If those basic checks don't resolve the issue, it's likely a problem with the furnace itself that will require a professional HVAC technician to diagnose and repair.