wont refund my ****** money

I was shopping at a store, Nothgate, my bag got ******. I only realized 30min later. they had already ****** R3500 out of my account using my pin. at two places. FNB will not refund me because of this. How did they know my pin????? it wasn't in my bag or phone!!!! I have video evidence that I did not leave the store and I called from the same store to the ***** line to cancel as soon as I realised. If I do not get my ****** money back I WILL change banks and advise everyone I know who is with FNB to change too.