On our 560 the economy gauge is operated by a vacuum line from the instrument panel to (I think) the EZL. Look for a loose vacuum fitting under the hood, perhaps near the left fender, depending on the layout of your engine compartment. If not at that end, it may have popped off at the back of the guage, pull the cluster and have a look.
'02 C230 Kompressor
'89 560 SEL "Frau BlueCar" (retired April 2004)