I don't use phpBB3 often but here's some stuff I've compiled just in case.
If one has forgotten one's admin-level password, admin access can be (re)gained by following this procedure:
In phpMyAdmin, run the following, which will create an admin user named Admin1 with a password of admin. From that point you should be able to get into the ACP.
INSERT INTO phpbb_users (user_type, group_id, username, username_clean, user_regdate, user_password, user_email, user_lang, user_style, user_rank, user_colour, user_posts, user_permissions, user_ip, user_birthday, user_lastpage, user_last_confirm_key, user_post_sortby_type, user_post_sortby_dir, user_topic_sortby_type, user_topic_sortby_dir, user_avatar, user_sig, user_sig_bbcode_uid, user_jabber, user_actkey, user_newpasswd) VALUES (3, 5, 'Admin1', 'admin1', 0, '21232f297a57a5a743894a0e4a801fc3', 'admin@example.com', 'en', 1, 1, 'AA0000', 1, '', '', '', '', '', 't', 'a', 't', 'd', '', '', '', '', '', '');
Change your table prefix if it is not phpbb_
As soon as you have done this, use the temporary admin account to change the details on the original admin account, then delete the temporary account. This is because:
To remove the account you will first need to remove founder status from it: ACP > USERS AND GROUPS tab > Manage Users > Admin1 > Overview > Founder = No