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Does the charger for the GPS plug directly into the GPS? if so, try powering it On, that way, while connected to the charger.
If the battery has been lasting less long lately, and the device is at least 1 year + old, then likely the battery is quite simply starting to fail - or, has failed as of today.
You will need to replace the battery in the GPS... would you like to try to do so yourself?
Or would you prefer to have someone else, or Garmin service, do it for you?