Ok, it seems to be that time of year again so bear with me while I swith gears from A/C to heat.
Check the furnace compartment for a manual reset for a flame rollout device or a hi-temp cutout device. Use a Volt/Ohm meter to see if your pressure switch is open. If so, check to see if there are any blockages in the exhaust. Possibly a wasp nest or something of that nature.