Can you register a new account? If you can, you can change the usergroup to admin with phpMyAdmin. Then you can assign your old accounts new passwords in admin CP
I had also tried it. I changed with phpMyAdmin the settings to allow registrations (I've turn them off), but trying to register I got the message the registartions are off.
Somehow I got a feeling that all changes that I did with phpMyAdmin had no effect. Unless (as it was already midnight), if I was working on another database with phpMyAdmin