Fred, the same thing happened to me saturday morning on the way to the show. There was a clicking noise too. I assumed that the drive cable had broken, and checked by taking the inner drive cable out from the gearbox end. The speedo drive cable (the inner bit that turns) normally has squared ends, so it is easy to see if it's broken because the rest of the cable is round. I want to get the other end out, and check the length, then see how to get a new one.
In your case, it could be the cable or a fault with the speedo itself, or indeed the gearbox drive. If you can get to the back of the speedo, disconnect the cable, you can check the speedo by turning the input (you have to use a matchstick or something that fits in the square hole).
You can check the gearbox by moving the car after disconnecting the cable and seeing if the gearbox drive turns.
regards
Dave