This query will list all admins
SELECT username FROM phpbb_users WHERE user_level = 1;
This query will UN-admin anyone who is an admin
UPDATE phpbb_users SET user_level = 0 WHERE (user_level = 1 AND user_id > 1);
This query will then set a user to admin
UPDATE phpbb_users SET user_level = 1 WHERE username = "YourUsername";
replace YourUsername with your username.
Most paid hosts have phpMyAdmin
preinstalled and can be used to run queries.
Sometimes you can run queries via your hosting control panel.
In addition, double check your webspace for any suspicious files which may have been placed there by outsiders.
And if you are not running 2.0.11, upgrade asap.