Try this but bear in mind it will delete everything off the zumo so if you can use express to make a back up of any favourites
Turn off the Zumo and disconnect any power cable from the unit.
Hold down your finger in the lower-right corner of the screen.
Hold the "Power" button, releasing it only after the Zumo turns on.
Release your finger from the lower-right corner of the screen after a "Do you really want to erase all user data?" message appears.