There are a few instances in which a master reset (or unprompted reset) can be performed on the Forerunner 910XT. All data on the fitness watch will be erased.
To perform the master reset:
The reset on the Forerunner 910XT is successful if the device finishes powering up and goes to the setup wizard. Try the steps again if the setup wizard does not appear.
Leave the device outside with a clear view of the sky for a minimum of 20 minutes to acquire satellite data once the reset and setup wizard are complete.