When I click the Challenge tab to fight other dragons, the tab is blank. All the friends that were listed under that have disappeared. What do I need to do to get that working again?

@Allie-Peabody I am going to guess that you are on an IOS device. For some reason, from time to time, when Dragon City updates, IOS resets some settings. Even if not on IOS, this is where to check.

You will need to reset your Facebook permissions. Upper right hand side of page go into your settings then follow along with this screenshot:

When you bring up your Apps, select Dragon City. It will bring up the game permissions. You need to allow the game to access your friends / contacts:
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EDIT: @noh8red39 Please do not double post on the same topic as this is against forum rules and considered spam and result in banning. Removing your comment, so that all responses can be posted to your first post.