I changed the username in the user manager and don't remember what username I selected (created several users that day). Long story short, I cannot login as the super administrator anymore (there are other people authorized as managers that can still login, so all is not lost).
How can I get into my joomla site again as the super administrator?
When you're looking through the tables in the database "jos_users", the first admin account created at install is id=62. That might help you find the username.
The text is german but its very easy to use:
You upload the file to the root of you J-installation. Then call it directly via you browser.
It resets user and pw to "admin".