A few questions.
1. How often did you use said account?
2. Did the account look like a shell, a main or somewhere in between?
3. Was it originally yours or someone else's?
4. Did you often buy a commonly used item ie: codestones? I can't say for certain but with friends getting accounts stolen lately, I think cookie grabbers might becoming more common again.
5. Do you own both the email used to create the account and its current one?
6. Did you have anything that would make you a target? Like were you showing off the stamp and other valuable items in trades/gallery/auctions?
7. Are you taking precautions on other accounts you have to minimize the risk it happens in the future? If so what are some precautions?
Feel free to skip any of these questions, I'm just thinking we may be able to help you more if we knew more about it without giving us information that would leak your account name.