Guestbook hacked? Recovery ideas?

My friend's guestbook (this used to be my domain; I do the maintenance for her, though) has been "hacked" -- it renders in Polish (?) and neither she nor I can get in with the login set up for her. I was able to download a fresh copy of the guestbook. I didn't want her to lose her guest posts, so I exported a *.sql copy of the original database. I then created a new database and new user, modified the config.inc.php file. It choked on the install.php script so I just went into PHPMyAdmin and imported the *.sql file back.

The Polish language was still there so I went in and deleted any references to any language other than English in the /lang folder. So now it's back to English again but it wouldn't let me logon. I cut and pasted the book_auth data from the freshly downloaded guestbook.sql file and pasted it into my database. I can logon initially, but when I click on ANY link in the Admin Panel, the login screen appears and it won't let me login.

Any thoughts? Plus I get a weird error when I point to the guestbook URL:

http://www.z-gal.com/zguest/

Help! Also, any ideas how this happened? I don't think it was hacked necessary. But what else would explain its sudden conversion to Polish?

The link doesn't work and when I go too the root (http://www.z-gal.com/)
I get this error:

Warning: main(./conf_global.php): failed to open stream: No such file or directory in /home/z-gal/public_html/index.php on line 93