-In your vbulletin istallation package, there is a folder called "do_not_upload", in which you wll find a file called tools.php.
-FTP this file to your forums admincp folder.
-Run this file from your browser by going to h**p://yourforumurl.com/admincp/tools.php.
-Go to the MYSQL section and select the "Reset Admin Access" option.
Piece of advise: if this hack was done by someone who was a stranger to you, you may want to consider renaming your admincp and modcp folders (with the corresponding changes in config.php) and password protecting your admincp folder via .htaccess when you get back up and running. If it was done by someone to whom you gave access ... well, you already know what you have to do.