I doubt they will give you an explanation of the fraud. I had a similar experience once and they credit card would not give me any information. The main thing is that you haven't lost out and your credit file has not been affected so, as others have said, you must check this. I know it's worrying and frustrating not to know what happened but I'm not quite sure how it would help you to know and I can see from their point of view to say nothing rather than say something that could breach confidentiality or rebound on them.