Hi,
Have you tried resetting it?
There is either a reset button on the bottom (it may be hidden by a sticker) or if it is not there then try sliding the power button on/off across and holding it for 8 seconds to see if this resets the device.
If neither of these work, try clearing the User data in the device by holding your finger on the lower-right corner of the Nüvi screen while turning it on. Keep your finger pressed until the pop-up window appears. Touch yes. All original settings will be restored. ''Be aware that any items that you have saved will be erased''.