Hi
Disconnect the speedo cable at the gearbox end. Turn the cable inner whilst an assistant keeps an eye on the speedo to see if the needle moves. If the needle moves suspect the speedo drive located in the casing at the front of the gearbox. If the needle does not move suspect the cable or the speedo. Check the cable first, take the cable out completely, check for wear and seizure, turn the cable inner at one end and see if the other end moves freely. Assume the cable is ok if it moves freely and that then just leaves the speedo itself.