If you are an EU Hearthstone (Free) player, then you probably already know that the client has been facing some issues the last day or so, especially with missing cards and adventures. Originally, Blizzard asked players to reset their Battle.net account passwords since it could be one of the ways to fix the issue. That apparently didn’t fix the issue, so now Blizzard has announced that the actual issue has been resolved and there should be no cases from now on. However, if you are one of those who are still affected, you can’t fix it by yourself. Instead, you have to contact Blizzard support team directly so they can fix it for you. To do so, you should click on this link so you send a support ticket.
Once you contact Blizzard and Blizzard resolves the issue, you should go back into the game and see if there are still decks with a red X on them. If there are, remove any one card from them and save the deck. Then go back into the deck, add the card back, and save the deck again. That should resolve the issue and you should once again see your missing rewards. Let me know in the comments if you still have issues after following these instructions.