This happened to me just last month. Xbox Live customer service was very helpful; I just had to give them my account name and the serial number of my Xbox, and they were able to see immediately that their system had flagged my account as being the victim of what they called "unauthorized access". They locked my account down and told me that I'd have my money back as soon as possible.
Now, the downside is that "as soon as possible" turned out to mean "two weeks later", but by them, I was just glad to have the money back and the whole mess behind me.
Now, the downside is that "as soon as possible" turned out to mean "two weeks later", but by them, I was just glad to have the money back and the whole mess behind me.