Here are a few things to check out first. If the furnace filter's surface is soiled, inspect it and replace it. Regular furnace filter replacement will help you avoid airflow issues. Because not all of the hot air produced by the furnace passes through the vents and into your living spaces, closed or blocked vents can also make it seem as though the furnace isn't producing enough heat. Verify that the louvers are open on every vent. Consider removing the vent cover and cleaning it if the louvers are stuck closed.
Also, make sure the gas valve is open by checking it on your gas supply line. Check your tank to make sure there is enough fuel if you use liquid propane or oil for heating; if not, you may need to arrange for a refill. Check with your utility company to make sure there haven't been any service interruptions for natural gas.