It sounds like you have been inside your machine...did you disconnect the antenna cables from the Airport when you were inside? Could they be loose and not properly snapped into place? Regardless, first I would try re-seating the antenna cables. If that doesn't work, try reversing the sockets they are plugged into. If that doesn't help, replace the Airport, and make sure to get a wireless-N model (AR5BXB72), since you probably only have wireless-G at this point. I just found one on eBay for $12.99 shipped.
If that doesn't work, you probably have faulty antenna cabling. In all honesty, the cabling is the most likely culprit overall, but re-seating the cables is free and a replacement card is very cheap, so it's definitely worth trying those things first.