If you think you have just temporarily misplaced a card, we recommend that you freeze it, so that you have the option of unfreezing it when you (hopefully!) find it again. Go to the "Cards" page, select the card you want to freeze, and choose the "Freeze" option.
If your card was stolen, or you think that it's lost and you will not be able to find it, please go to the "Cards" page, select your card, click "Review card settings" at the bottom of the cards panel, and choose to cancel your card.