My top teeth have now moved backwards. How Can I Fix my Teeth? (photo)

When I was 16 (Now 20) I had braces put on my top set of teeth due to them being a bit wonky. I had my braces taken off a year ago this month. When I had them taken off I wore the retainer for the first month, then my dog decided to chew it, never got round to arrange to get a new one from the dentist. Since having them off and stupidly not wearing the retainer, my top teeth have now moved backwards. So now, my bottom set of teeth cover the top set of teeth. What will fix my teeth!