Several different issues cause a central heating system to stop working. For instance, when the power is out, most systems no longer provide heat. Even though a forced air system uses natural gas as fuel, it may require electricity to operate the blower. If too many lights and electronics are on in the house, power overload can trip the electrical circuit breakers and affect the central heating system.
Blocked air ducts or air leaks can be the culprit, particularly if most of the house has heat but one room remains cold. Rodents and insects build nests in warm passages and block air circulation throughout the home. Over time connections deteriorate, and this allows the warm air from the furnace to leak into unfinished spaces of the home. In homes with radiators, air trapped in the radiator sometimes prevents steam or hot water from moving throughout the unit, leaving one or more rooms cold.
In gas systems with a standing pilot light, the unit must burn continually. Wind gusts or back drafts down the flue sometimes extinguish the pilot light and prevent the furnace from working properly. Check the unit to ensure the pilot is lit. High efficiency systems use an electric ignition to light the pilot on demand and start the burner automatically.