If the admin user is still defined, the simplest option is to change the password is in the database to a known value. This requires that you have access to the MySQL database using phpMyAdmin or another client.
This Tutorial is also for users who can't login in the Backend or have lost or forgotten their Password.
1.Navigate to phpMyAdmin and select the database for the Joomla! site in the left-hand drop-down list box. This will show the database tables on the left side of the screen.
2.Find and click on the table with "_users" appended in the list of tables (note: you may have a prefix that is not jos_, simply go to the _users table for your prefix).
3.Click on the "Browse" button in the top toolbar. This will show all of the users that are set up for this site.
4.Find the user whose password you want to change and press the Edit icon for this row.
A form will display that allows you to edit the password field. Copy the value
into the password field and press the Go button. phpMyAdmin should display the message "Affected rows: 1". At this point, the password should be changed to "secret".
6.Log in with this user and password and change the password of this user to a secure value. Check all of the users using the User Manager to make sure they are legitimate. If you have been hacked, you may want to change all of the passwords on the site.
This Article is a collection of the original Joomla Docs.