Card found in arena showing not found

Solution 1:

Many scenarios could have happened here:

Your game crashed

If a game crashes, the previously stored data could be lost. If only your recent data was deleted, unfortunately, your data, cards, and arena are unrestorable.

I know this can happen from personal experience with games like this and others as well.

Unlinked account

Your account could have been unlinked from your phone, and therefore Clash Royale. This can happen if there is an insecure or slow internet connection, and you then must relink your account and select "Yes" when prompted to restore data.

Purchases restored

If you bought your cards in the shop, with real money, your account might have restored purchases and undone every purchase on Clash Royale. This could happen by accident, or by a supervisor of your account (parent or guardian). You will have to re-purchase at that point and hope to be as lucky as you were when you got your legendaries.

Server failure

If everything above does not apply, the only other option is that a server owned by Supercell lost your data. If this is true, contact Supercell as soon as possible so you will be able to recover your data.

Help me help you

It would be much easier if you were to tell me your type of phone (iOS or Android), show screenshots of the game and your missing cards, and tell me if you have purchased any in-game items on Clash Royale in order to narrow down the scenarios.

Remember to check if you are logged-in, if your purchases are restored, if your game has crashed recently, or other unusual behavior, and comment it to make it easier for me to find you your solution.