Go to a real store nearby you and explaint he situation - did you preorder online or in store? If ina store you need to visit the one you preordered at and they can definetly refund you. If it's online I still recommend going into the store, they're likely going to tell you that you have to contact customer service but you're young so I know that it might be confusing - go there and ask anything you'd like, like why was it cancelled, how is this going to work out, etc.
Gamestop is only as bad as it's employees, the people who work there are just that, people. I know some people have had bad experiences but my local Gamestop is a wonderfully run place filled with very nice people and the manager is super stern and gets stuff done. For all you know it could have been a system error, or maybe since it was a prepaid card they tried to do card authorization (where they check to make sure the card is good and has money to pay for the item) and it bounced for some reason.
But yeah! Just visit a store and ask for help.