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 a problem with the thermostat. First, check the circuit breaker and make sure it hasn't been tripped. If the breaker is fine, check the air filter and replace it if it's dirty. You should also make sure the thermostat is set to 'heat' and the temperature is set higher than the current room temperature.
If those basic checks don't resolve the issue, there may be a more complex problem that requires a professional HVAC technician to diagnose and repair, such as a malfunctioning ignitor, gas valve, or blower motor. It's best to call a licensed HVAC contractor to properly troubleshoot and fix the furnace, as attempting complex repairs yourself can be dangerous.