This issue has been resolved with the most recent update. Please update your game.
Due to a new issue; some images, graphics or UI/interface may be missing or appear invisible in the game. Dungeons may also be unplayable and freeze after a battle and it may be difficult to access the Kingdom Merlantis. This is caused by assets downloading incorrectly.
Fixing the problem
Currently, the only workaround is to reinstall your game. Ensure you account is linked before doing so - for more information see this article here. Next, reinstall the game from the App Store and sign in after playing through the tutorial to recover your game data.
Note: Players have reported that reinstalling does help, but you may need to try a few times to fix the issue. The Gems of War team is investigating this problem for correction in the next update.