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Try unplugging the Nuvi device, and leaving it without power (the battery removed also, if it is easy for your to do so) for at least 2 hours.
See if that clears up the problem. If it does not, then you probably have a 'dead' device and it will need either repair or replacement.
If it is fairly new, then its repair might be covered by Garmin and done at their expense under warranty terms.