I've had it a few times, and how I respond depends on what I am doing and where I am. Sometimes I just shut out the world - I get room service, or visit a local shop and then vegetate in my room all evening, with a good book, music, or something good (if there is anything) on the tv. Sometimes calling home helps, sometimes it makes it worse. If I listen to music, I have a group of cds which I know cheer me up.
My other reaction is to get very busy - I try/visit something new, so that I'm too busy worrying about getting there and doing the tourist thing. And very often the new sights and sounds do envigorate me again. Or I try and just enjoy where I am - so in Tokyo, look at everything that is Japanese and unfamiliar, and appreciate the fact that I am able to be there to experience these and my friends are very jealous of me
I've always found it a mood thing, and if I can break the mood, it will pass. Best of luck!