I just went through the same problem. The very first thing you have to do is check the rheostad or dimmer controller which doubles as the miles reset in your instrument cluster. It is the left "handle" in your cluster. Turn it right and left a few times. If after doing that, still does not work, you may have to replace the fuse behind the cluster or the rheostad itself. My car a 86 300E does not the fuse behind the instrument cluster (it looks like a lighbulb, red in color?), MB started installing the fuse in 87's but, I do not know if this applies to your car, since is a 91.
I bet you turned the mileage reset handle by mistake during the day. Try the first suggestion. Good luck!