Well it depends if someone has actually taken over your account or not.
Few things I suggest.
- Change your password from some other computer
- Download a fresh copy of The home of Spybot-S&D! and run it on your computer
- Make sure you do not use other computers to log in and check your email.
As a computer security expert, I know that it is very easy to steal passwords and if someone has installed keylogger then they will keep on using your information. So make sure that your run anti-virus and anti-spam on your computer at least once a week. And in case if you do see some trojans, keyloggers etc,. Remove them and reset all of your passwords immediately.
p.s. : Now go change your password lol