If you can still access the account, check to see if the hacker has changed your email address. The email address currently associated with the account will show under "E-mail settings" on the Settings page.

If they have changed the email address, change it back to your own, and sign out of the account. Then, reset your password. If you're unsure how, check out our tutorial: How to: Reset Your Password

After doing that, you will be sent an email to the address you entered containing a link to reset your password. You will be given a new, temporary password, which you can change afterward on your Settings page, under "Change password".