The password can be reset with the aid of phpMyAdmin in cPanel.
! If you can't get into the Joomla admin and you know your password hasn't changed you may have been hacked.
The directions below will get you back in, but your site should be checked over for other problems.
Here's a good video: Recover your Joomla! Administrator password using phpMyAdmin
Another Way:
Run this SQL on the Joomla database using phpMyAdmin:
UPDATE `jos_users` SET `password` = MD5( 'new_password' ) WHERE `jos_users`.`username` = "user_name" ;
Where 'new_password' is the new password and "user_name" is the user name.
In this case the password should be unencrypted as it will be encrypted by the MD5( 'new_password' ) in the SQL.
This will create a password which will allow you to login. For best security the password should be re-entered in the User Manager to add the "salt". The "salt" is a random string added after the MD5 encrypted password (with a colon separating). (md5encryptedpassword:salt)