If your furnace is not blowing warm air, check your filter first. Usually a furnace stops working because the filter is so dirty that it is blocking the air flow and that it won't let the proper amount of air flow through it. Go to your furnace and remove your filter. If it is completely blocked with dirt, dust, etc. you have probably found the culprit. Remove the filter, and depending on the type of filter you have, either wash, vacuum, or replace it. Don't worry if you don't have a replacement filter handy, as you can run you furnace without a filter for a little while. Don't run it without a filter for more than 1 or 2 days, though. After you have removed and/or replaced the filter, turn the furnace off, then turn it back on. This will usually "reset" the "brain" (control board on newer furnaces), because the control board or "limit control" shut the gas off when it started overheating. A furnace will overheat when trying to run with a very dirty filter. If the furnace still doesn't work, a over temperature limit sensor may have tripped. At this point, a professional heating technician will probably be needed.