If you still have access to the app, you should 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 – no need to order a new card.
Sure your card has been lost or stolen? You can cancel it and order a replacement in the app – just go to the Card screen and tap Cancel & Replace Card. You can re-add the new card to Apple Pay or Google Pay as soon as you've ordered your replacement, so you can carry on spending before the physical card arrives.