Furnace power switch – Was it turned off by mistake? In most homes, it looks like a light switch in the mechanical room, often above your light switch (closer to the ceiling).

Thermostat batteries – some thermostats take batteries. If the screen is blank, the batteries may be dead.

Thermostat – is the “System” section on your thermostat set to “Heat”? It may have been accidently changed to off, or cool, etc.

Furnace filter – has it been changed recently? Most filters will have instructions on how often your filter should be changed. The filter life varies from filter type, manufacturer and whether you run your furnace fan continuously.  Inexpensive, fibre-glass filters need to be changed every month.

Furnace exhaust/chimney or air intakes – check outside where your air intake or exhaust (high efficient are usually on the outside wall, but can be on the roof as well). If they’re on the roof or if you don’t have a high efficient, check that the snow cover is not obstructing the exhaust/chimney or air intake. If they’re on the side of the house, check that there is nothing stored too close: bushes, lawn chairs, snow build-up, something covering them (screen or fabric), etc. If you there is, you may see a lot of frosting around them. Remove the obstruction.