Hi Moonchild,I agree with everyone else, see a doctor as soon as possible. It may not cure you, but it's a good start.
I've been having problems with my left hand for about 3 years now, and my right hand for about a year. My problem developed over time by playing too much (more than I was used to, anyway). I can relate to not being able to play.
Was there any warning that something was wrong? It sounds like it happened suddenly for you. I would guess it's not arthritis, although I'm no doctor. Arthritis would probably come about gradually. Bursitis is possible but I wouldn't worry too much. Something as simple as a sprain can cause all kinds of strange things to happen. A lot of times, things like that come and go and there is really nothing to worry about.
The problem could be anywhere -- your hand, arm, neck, back, hips. Have you had any pains developing lately? A pinched nerve is also possible.
There are a few things I could suggest from my own experience that might help alleviate the symptoms, but I'd get it checked out first. I wouldn't want to mislead you by giving bad advice when I don't even know what the problem is.
One thing I'll stress: IF IT HURTS TO PLAY -- DON'T! You may cause more serious damage. Cancelling the gig would be a good idea in my opinion.
Take it easy, Jason