For starters we need the URL of the blog that has been "hacked" to give you accurate advice
In the mean time here is some general advice:
The only “hacked” sites I have seen here have been people getting the password somehow to a site so you do want to be careful how you log in and use a tough password.
You should also check to make sure that someone has not added a new user to your site. Problems have also happened when there was more than one Admin. and an Admin left on less than graceful terms.
Dashboard >> Users
Many times a users email is hacked or the password guessed and the email account is used to get access by requesting a password reset – you need to change your email password and make sure they nobody has been added to your email account.
You should also run a virus check on you PC to make sure your PC does not have a security issue that has compromised your email or WordPress.COM log-in information (also your bank etc. if you do online banking or retirement accounts)
There have also been a few Posts on “hacked” sites and it was because someone got the Post by Email address and using the Post by Email to send in new Posts, if you have Post by email disable the Post by Email and regenerate the address. Spammers have scripts the generate email addresses and they sometimes can get a valid address for a Post by Email address.