The speedometer in my 86 f350 stopped working. One day it worked, then nothing

Hello, The cable or gauge is broken internally. Disconnect the cable from the transmission end, spin it back and forth to see which direction is correct (watch the gauge) and use a drill to spin it in that direction. If the speedometer jumps, your speedometer drive gear in the trans is bad. If the speedometer does not jump, remove the speedometer cable from the back of the speedometer, and see if it turns when you spin the other end. If it turns and is not damaged, bad speedometer.