The only real way to fix this is to unban everyone. Depending on your version you'll need to do 1 or 2 queries, probably using phpmyadmin or something like that if you can't access the control panel.
First query:
update ubbt_USERS set USER_IS_BANNED='0'
If you're running 7.1.x you'll also need to run:
delete from ubbt_BANNED_USERS
Unfortunately, this will mean everyone will be unbanned including those that were banned prior, but that's really the only way to recover.
