Open your database admin page (PHPMyAdmin or whatever you use) and see if the tables are still there.
Btw, that error shows that you deleted the Guest account.
These queries should fix that:
DELETE FROM `users` WHERE id=1;
INSERT INTO `users` (`id`, `group_id`, `username`, `password`, `email`, `use_avatar`, `email_setting`, `save_pass`, `notify_with_post`, `show_smilies`, `show_img`, `show_img_sig`, `show_avatars`, `show_sig`, `timezone`, `language`, `style`, `num_posts`, `registered`, `registration_ip`, `last_visit`) VALUES
(1, 3, 'Guest', 'Guest', 'Guest', 0, 1, 1, 0, 1, 1, 1, 1, 1, 0, 'English', 'Oxygen', 0, 0, '0.0.0.0', 0);