Stolen/hacked Apple ID/GameCenter

Hello. I've been playing the game "Clash of Clans" for a very long time. A few days ago, someone managed to log into my Apple and GameCenter ID, wich made me getting a notification. I changed the details immidiately, but but the person now still has acces to my Clash of Clans account. He is able to do anything and even log in when I play. I've put real money on this game, and I am level 109. I don't want to loose anything, but I really need to get that person away! He is ruining everything!