Credit card not showing up in Wallet / Apple Pay

I just went to use my phone to pay for something (as I do almost every day), but when double tapping the Home button at check out nothing happened. I checked in Settings and in my Wallet, and it says I can "Add a credit / debit card", so my existing card seems to have disappeared. However, when I go to add my card details, it tells me that my card is already in my Wallet...?

I have searched for assistance and have tried a bunch of other peoples' suggestions (e.g. reset my passcode, turned my phone off/on again, deleted the card on icloud.com and reinstalled it), but I am still having exactly the same problem where the card doesn't show up, but when I go to add it, it's "already in my wallet".

Please help!