[CrazyJ Cena]

Suppose you are only admin and you've banned yourself, then?
Freaky, but accident happens ...

Go to your hosts control panel and launch phpMyAdmin.

Run the following commands:

Code:

DELETE FROM mybb_banned WHERE uid=1

next ...

Code:

UPDATE mybb_users SET usergroup=4 WHERE uid=1

You are unbanned and assigned to admin again!!!

Note:
Assuming your UID is 1 (default for first member : admin).
Assuming your database table prefix is mybb_ (default), if not, change what is.
Assuming your usergroup is 4 (default for Administrators), if not, change what is.