Check the drive cable first. If that's OK, watch the end of it while turning the front wheel to see of the cable turns. If not, the drive mechanism at the wheel may have broken. In this case, do not ride the bike untill it is replaced and all the broken bits (if any) are cleared out because they can damage the bearing seal. If the cable does turn, make sure the (not quite) rightangle drive is tightly attached to the back og the speedometer. If it is tight, remove it and check the gears. IF OK there, the speedometer needs to be tested.
