Probably replace the speedometer cable. The cable connects to the side of the transmission, then runs up the bottom of the car to the firewall, then into the back of the instrument cluster.
The cable is turned at the bottom by the transmission, and at the top end the cable turns the mechanism of the speedometer.
It's probably 1 of 3 things: The speedometer cable is broken, or the cable is loose at one of the two ends, or the speedometer gauge is broken.
You can take the instrument panel off and look at the end of the cable where it sticks out. If you do that and drive a short distance, you can see whether or not the transmission is turning the cable.