I had the same problem on my W123, and it turned out that the cable had come loose and simply backed out of the speedometer head. I had to fabricate a pair of those instrument panel puller thingies from a coat hanger, but it was an easy fix. My philosophy is to check the simple things first; good luck.

Best Regards,
Rob

__________________
Robert Fini

'12 ML350 BlueTec, 111k
'06 E320CDI (x2)
'05 T1N Sprinter 2500/Pleasure-Way Plateau TS, 72k
'97 C36AMG, 325k (son's)
'94 E320T, 249k
'93 190E Sportline LE, 168k (daughter's)
'84 190E-2.3/5spd (Stage Rally Racer)
'66 230 W110 Sedan (Barn Find, Vintage Racer build in progress)