You might see the following error when trying to deploy either a test or final package build of your game to your device:
This means that the WP8 app deployment tool cannot see your phone becauswe it believes the phone is PIN locked. If you do not have a PIN set up on the phone this usually means the phone is sleeping/not on the home screen. If simply waking the phone and ensuring it is on the home (tiles) screen doesn't fix, try unplugging your device from USB and plugging back in again while ensuring the device is awake. Obviously, if a PIN is required by your phone you will have to enter it.
You don't have to restart GM for the change to take effect and your .xap should now deploy fine again. If it still does not, please restart your phone and your PC before trying again.