If you have lost the administrative password to your Joomla application, you will need to manually reset it.
The passwords are stored in the Joomla database, but in an encrypted or "hashed" MD5 format, so you will not be able to recover it.
This guide from Joomla will provide you with the best instructions for resetting it.
Essentially you need to connect to PHPMyAdmin via yourdomain.com/pma/ and login with your Site Manager user name and password. PHPMyAdmin can be installed through the Site Manager >> Install & Manage page if you do not already have it running.
From there navigate to your Joomla database. Navigate to "jos_users" [the jos_ may be different if you have a custom table prefix]. Modify the admin user and type in the password you would like, then select MD5 from the dropdown menu.
Save the entry and attempt to login to your Joomla admin. Once connected it is recommended that you update your password again from inside Joomla to ensure that it is secured.