sounds like the cable has popped off from the centre of the wheel. the little clip that holds it on are prone to breaking easily. either that or the cable has come undone on the back of the actual dial but thats normally unlikely because its a screw fitting.
when my trekker had its last m.o.t it came out not working coz when the bloke was checking the breaks on the ramp and turned the wheel he must have turned it to hard, pulling the cable and snapping the clip. if you can't get a clip u can always use sum fine wire to wrap round the end of the cable and that should hold it until u can get one. thats wot i did and it held brilliantly until i found a new clip. hope this helps.
