I am hoping you can help me.

I went to ban a member in UBB.threads™ 6.5.5 and instead of the 'selected members' I must have had 'all in search' clicked so when I banned the account I managed to ban all 15,209 members! This means of course that I have banned all the administrators including the webmaster.

Whoops.

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.