Disconnect the Sky box from the electrical supply. Press and hold the back up button of the Sky box while re connecting the electrical supply. Keep holding the back up button (up to around 20 seconds) until the screen goes black with white text ‘Updating system software. Do not disconnect’.
Wait 10-15 minutes until the text disappears, at this point the box has gone into standby mode. Wait a minute or so and then, using your remote control, switch the box on.