Were your other dash guages working? If not it is most likely a fuse. I had the same problem. Started up the car and none of the dash guages worked. Got a spare fuse from the glove box and it was fixed. If everything else is working it is possbile there is a loose connection at the back of the guage cluster, or it could be one of a few other problems, none of which I really know anything about.
1995 E420 - retired
1986 420SEL - retired