Yep if you can't login to admin its time to browse phpmyadmin settings table settings 8-10 and blank those. While your in there go to setting 171-173 and reverify your cookie settings. If your using vbulletin no doubt your cookie prefix is empty and you need to set this by entering your cookie settings here. They are in your
vb admin panel and the prefix is in the vbulletin config file