Is your site static pages (HTML) or do you get user input and dynamic pages (PHP)? If dynamic pages, there may be a security hole somewhere in your scripts. In either case, the hackers may have stolen your password. You should be running a spyware scan on your PC to see if there is a keystroke logger installed, that is passing on your passwords to others. Change all your passwords immediately, go through your site's files with a fine-tooth comb looking for stuff someone else added, run a spyware scan, and if anything is found, change your passwords again after the logger is removed.