Fraudulent activity has taken place on my account. How do I find out who made this purchase...

I have checked my Microsoft account but have no games showing as purchased, priced at £49.99 but my bank statement shows this charge. I do have my Xbox subscription and a game I purchased yesterday showing which is correct. How do I find out who made this £49.99 purchase what it is for? How do I go about getting my money back.

:)