Well, it could only be four things--the speedo may be stuck, the cable may have broken, the cable may be detached at the speedo end or the tranny end, or the flange on the tranny may be loose. You should be able to narrow this down pretty easily by conducting a couple of tests. I'd first check out whether your cable is turning when the car is in gear and the rear wheels turning. Do this carefully while the rear wheels of the car are lifted off the ground with a set of jack stands. May help to remove the wheels (tires) themselves when you do this. This will eliminate a couple of things right away.
Georg
