Do you have Mapsource program/maps installed on PC? Does the PC/Mapsource recognize the Zumo when you plug it in? You should be able to reinstall the map files. The actual mapping/routing functionality is embedded in firmware and not subject to erasure by us idiots, so replacing map files should work. Of course, I may be full of it.
Worst case, call Garmin Support, wait a bit to get a tech and have them walk you through it.