The information provided is from here and explains why it does that. "Garmin Mass Storage Mode devices no longer communicate with a computer with the Garmin Protocol but instead present themselves as a USB mass storage device. When a USB cable is connected between a Garmin Mass Storage Mode GPS device and a computer, the GPS device enters mass storage mode." so you could try the reboot by :
1) Disconnecting the nuvi from the charging cable.
2) Press and hold the battery meter icon for 8 seconds. This puts the nuvi into "system test" mode.
3) Plug in the power cable.
4) Click "Exit." The nuvi should now reboot, but remain in GPS mode!
The only other thing I can think of is that you might require a new cable, if this just came up. Good Luck and I hope this information helps.