@paxx:
If it were a username/password problem, the error message would be accordingly, and different from the one posted here. So, that's not it.
@buyalong:
What exactly in "user permissions" did you change? Your usergroup? Or permission settings in "Global Configuration"? Either one can well lead to you locking yourself out from the backend, indeed.
In any case, if you don't have another Super User or other account with permissions sufficient to access the backend and reverse your changes, you'll have two options:
(1) Restore from a backup.
Seriously, one must assume that you've made a proper, complete and restorable backup of your site before attempting significant changes like the ones you made.
(2) Reverse the changes directly in the database, e.g. with phpMyAdmin.
What exactly this requires depends on what exactly you've changed, obviously.
Note: If you're not familiar with database operations and/or phpMyAdmin or whatever other tool you're using, you're in danger of causing even greater damage, and you should consider hiring an expert to perform the repair.
However, this is related to Joomla (and not CB or its plugins). So, you will likely get limited help here anyway, and might need to seek more help elsewhere, if needed, e.g. in the forums on joomla.org.