When you don't press any key on your Keycard Shell for a period of time, it will automatically turn off. You can turn it on again by pressing the side button on the left (recommended), or by slightly removing and reinserting the card.
By default the auto turn-off time is set to 3 minutes.
You can change it to the following values: 3 minutes, 5 minutes, 10 minutes, 30 minutes, or 'Never' by going to Settings > Shell > Auto-off time