You mean you've deleted the account or just changed your usergroup?? You've not been very clear. Assuming you've just changed the usergroup, run this:
UPDATE `mybb_users` SET `usergroup` = '4' WHERE `uid` = '1';
Change the 1 to your UID if it's not 1.
Thanks for your fast support!
Choose your database from the list on the left, and click the SQL tab along the top.
They won't be able to edit you because you're a super admin.
Who are you hosted with?? It's extremely rare that it's not installed, otherwise people have no way of managing their database.