Simply check to make sure your thermostat is set to Heat (if yours is a heating/cooling system)
Be sure the circuit breaker is on or that the fuse hasn’t blown. If so, reset the circuit breaker or have the fuse replaced.
The motor may need to be reset because of an overload. Look for a reset button near the blower motor’s housing and press it.
Turn off the power to the furnace at the main electrical panel or subpanel. Look for a fuse in the power switch, and if it is blown, replace it.
With a gas furnace, the pilot light may have gone out or the gas valve may be shut off. Check the furnace’s ignition.
If the furnace still doesn’t work, be sure the thermostat isn’t faulty.