The answer to 'my furnace won't turn on' is that there are several common causes, including a tripped circuit breaker, clogged air filters, issues with the thermostat, or a problem with the furnace itself. Symptoms can include the furnace not starting at all, short cycling (turning on and off repeatedly), or the blower fan running but the burners not igniting.
Some simple DIY troubleshooting steps you can try include checking the circuit breaker, replacing the air filter, and ensuring the thermostat is set properly. However, if the issue persists or you're not comfortable working on the furnace, it's best to call a professional HVAC technician to diagnose and repair the problem.