What causes fingers to go numb?

Carpel tunnel or nerve damage will often cause this. Go see a doctor.

Go and see a qualified medical practitioner (a doctor) if you are concerned about your health.

A few years back, my mother had the same problem. Her's turned out to be carpul (spl?) tunnel syndrome. Basically the cartiledge in her wrist was "choking" the nerves that led to her fingers. Because...

cold temperatures, ms, diabetes etc.

Anything that prevents blood from flowing through to the foot.