There are a few instances when it may be necessary to perform a master reset on the Edge 500. A master reset should be performed if the Edge is:
All settings, activities, and satellite data may be erased when resetting the GPS. Activities can be backed up to Garmin Connect. Refer to the FAQ article "How can I back up my personal settings and profiles on my Edge 500?" to back up personalized settings.
To perform the master reset:
After the Edge 500 is reset, it will walk through the initial setup. After completing this, leave the device outside with a clear view of the sky for approximately 15 minutes to acquire satellite data.
*If the device is not receiving a satellite signal, perform an AutoLocate before performing a master reset.
Garmin Connect - http://connect.garmin.com/?s=1