had the same happen on my '88 300e; youl have to pull out speedo unit out of the dash; while its out, you might want to replace bulbs and such; one youve isolated the speedo unit you maynotice that the speedo gear has derailed itself and just applying some pressure on the exposed end will snap it back.
Im noticing on mine that certain numeric combinations cause it to derail, methinks theres a tooth missing in there.

It is impoosible to work on the odo gears themselves without majorly damaging the speedo; all you can do is carefully re-rail the spindle or send to mr. vdo for professional help.