Look at obvious first
I am assuming that the speedo is still working? I'd check the fuse box for burned or broken fuse. Sometimes they look good but have a hairline flaw that isn't visable until you move it. I'd next pull the cluster and insure all electrical is plugged in. If it is, unplug them and reinsert them again. Could be that one or two pins of the connection have lost contact with it's counterpart.
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan