If your card is lost or stolen, you should lock your card as soon as you can. If you still have access to the app, you can lock your card from the Card screen so it can’t be used if it falls into the wrong hands. If you find it again, you can just unlock it - there’s no need to order a new card.
If you still can’t find your card, you can cancel it and order a replacement in the app. You can re-add your card to Apple Pay or Google Pay as soon as you’ve ordered your replacement card, so you can carry on spending before your physical card arrives.