Most of the PhpBB hacks exploit the code to inject information into the MySQL database. I would suggest looking through your site using either FTP or the File Manager in cPanel to see if there are any files that you don't recognize. You can also look at the database using PhpMyAdmin and see if you see any odd entries left over. I would guess the restore of the database you had run corrected that for now. You should update the software as soon as possible to keep them from returning to hack in again.