Either you got a bad bulb, or something popped in the ballast. Heat isn't an issue with the bulbs, they run in unventilated fixtures all the time. A bulb should burn for atleast a year and usually a lot more. It should still light up fine when it is no longer putting out usable light. Maybe borrow a bulb from someone to see if it's you bulb or ballast before spending any money. Also check all you connections and breakers.