I've had two occasions when I lost my sense of humor. The first was overwhelming burnout in college. The second was right after Sept. 11 (I live in the US). In both cases, the laughter started to return after a month or two, and gradually recovered completely. Maybe it will be the same for you.
Adair has a really good point. Indulging in cynicism might be just the thing you need. It worked for me when I was burned out. It feels really good, and although it makes things worse for a while, eventually you get tired of the cynicism and go back to laughing.
Good luck. (No sarcasm intended here.)