There are a few instances in which you may want to perform a master reset on your Edge 800, such as:
All settings, workouts, and satellite data may be erased when resetting the GPS. Workouts can be backed up to such training programs like Garmin Connect or Training Center. Refer to the FAQ article "How can I backup my personal settings and profiles on my Edge 800?" to back up your personalized settings.
To perform the master reset:
The Edge 800 is now reset. You will want to leave the device outside with a clear view of the sky for a minimum of 20 minutes to acquire satellite data.