My 920XT brick after update via Garmin Express.
I do this:
1. Hold "powerbutton" for 15 seconds. The device turns off.
2. Press and hold "upbutton" and "downbutton"
3. Press (not hold) "powerbutton" for 1 second (until device turns on)
4. After first beep, release "upbutton"
5. After second beep, release "downbutton"
Now is running fine.