Don’t worry! You can get the gift card back. Go to the purchase receipts section of the app. Find the gift card and click on it. Click on unarchive and enter the amount on the card (or if you’re not sure what the balance is, just enter $0.01 for now). Then, go back to the retailers section of the app. Scroll down to the retailer and click on it. Your gift card will be there again. If you’re not sure of the balance, go to the retailer’s website to check the balance, and update it in the app to help you keep track.
These same instructions will work for you if you've accidentally changed the balance of a gift card to $0, or if you have a gift card with $0 balance but still need to see the full gift card number for any reason. Any gift card with a $0 balance will not appear in the retailer's section of the app and will be removed from your "wallet". If the gift card has a balance of $0.01 or more, it will remain in the retailer's section of the app and it will also appear in your "wallet".
This is also good to know if you're returned an item to the store. Some retailers will refund your money back to the original gift card. In that case, just follow the instructions above to update the new balance of the gift card.