So did the light work OK before this happened? I just changed over my speedo on the weekend and had the fun of getting all the wires back onto the right pins, was a bit fiddly there.
I'd be looking for the wire and trying to get it back on, otherwise I reckon you run a rish with a potentially live wire (small voltage) possibly either touching the body somewhere or even tapping on another wire contact.
Do all the other lights work properley in the speedo cluster?