Do something about it, don't just wallow. Wallow for a bit if you like, but when being miserable's just - well, miserable - then take control of your life, it's so empowering. As people have said - watch comedies, listen to upbeat music, CHOSE TO MAKE IT BETTER. A really good thing is to do something vigorous - go for a fast walk, a swim, put some dance music on and bop about like a maniac. Exercise stimulates the adrenaline, serotonin, whatever it is, it just makes you feel better, takes your mind off it, shifts the blocked energy. I know what I'm talking about I've been there. It may not work for everyone, but it works for me and it's worth a try because melancholy may be deliciously gothic but it's not a patch on having a good time.
Good luck, MissyR