Try to clear your browser cache and reload the page. The arrow to move back to World 1 - 4 is still present. Once you did, you should be able to fight the Dark Lord in Gems Kingdom again.
Anyone else noticed that the drop chances for girls have been increased? With the start of the Christmas Event I got Eugénie, Jeanne, Sheheramazond and Winter Agate within a few days. That's a wonderful christmas present