For security reasons, you will not be able to retrieve your lost password. However, you will be able to reset / change your password by logging into phpmyadmin and running an SQL query.
Log in to your hosting account as an administrator and launch phpmyadmin. If you have multiple databases, make sure your Joomla! database is selected before clicking on the SQL option at the top of the screen.
Copy and paste the code below, making sure to change 'password' to your newly chosen password and your 'username' to your chosen name. By default, the username for Joomla! is 'admin'.
UPDATE `jos_users` SET `password` = MD5( 'password' ) WHERE `jos_users`.`username` = "username" ;
Click 'go' to submit the query.
Make a note of these details on a piece of paper and store it in a safe place. Your new username and password should now be working.