This happened to my girlfriend's hotmail a few weeks back, her account was hijacked (she was locked out) and they had attempted to sent spam emails to her contacts in order to retrieve details.
It is a good idea to look into what makes a strong password!
http://www.microsoft.com/en-gb/security/online-privacy/passwords-create.aspx
You may also find in your email settings that the hijackers may have used a secondary email to link to your account.
In order to prevent this from happening again make sure that your computer(s) are checked for viruses and just be aware of nasties on the internet.
http://www.switched.com/2011/02/24/what-to-do-email-account-hacked/
Don't worry too much though, it won't be a single person, chances are it's probably just a robot hacking millions of accounts and just trying to use yours to sell things.