I know this is a common issue and a lot of people want to know how to fix this. There are a few threads explaining how to reset the nav but require a remote. For those that do not have a remote try this first.
This works if you have updated your nav and need to downgrade back to original disk.
-First insert upgrade disk and go into nav menu.
-Then go into your nav settings and change your language, doesnt matter what you change it to just as long as it is different than what you currently have.
-The system will try to update and will more than likely fail, eject the upgrade dis