It could be what Gary suggested as well ("The other cause is usually nerve compression,").
I had a numbness and tingling in my left forearm, thumb and pointer finger. Diagnosed as nerve compression, I think it was the 3rd vertebrae. Saw a chriopractor - didn't work. Had to visit a physiotherapist for about 6 months. She put my neck in a sling with weights and cold compresses. Didn't work very well until she laid me down on a bed and pulled my head with her hands (yes. my head did not come off, make a wish ). I believed it happened when I was sleeping on my stomach. Hasn't reoccured since. Phew!
You need to get this diagnosed. Good luck.
Starkeeper

[This message has been edited by Starkeeper (edited 12-02-2004).]