Check to see if someone moved the thermostat switch from "Heat" to "Cool".
Check to see if the switch on the side of the furnace was accidentally left off.
Make sure the electrical breaker is ON.
Make sure there is not a problem with the gas service coming into the house.
Make sure the return air filter is clean.
Check the flue exhaust cap for obstructions; bird nests, snow build up, etc…
The above list of items occurs more often than you might think and by checking them ahead of time, could save you money ... although we would be more than happy to come out for a service call :-)
