Check your circuit breakers: If your furnace won't turn on, a tripped circuit breaker is a simple remedy. If that doesn’t help, check your thermostat settings. Your thermostat may have reset when the power went off, or it may be programmed to keep your home cool on a schedule. Next, make sure your furnace filters are in good shape. If your furnace isn't getting enough air, it won't turn on. A clogged air filter can obstruct airflow and make it difficult for your furnace to start. Also, check your pilot light. If you have a gas furnace, the pilot light will go out if the gas supply is turned off. Finally, check your blower motor. If your blower motor is broken, your furnace will not be able to push out warm air. If all else fails, call your local HVAC company to diagnose and fix the problem.