This either means that the username and password information in your settings.php file is incorrect or we can't contact the MySQL database server. This could mean your hosting provider's database server is down.
The MySQL error was: Access denied for user 'username'@'localhost' (using password: YES).
Currently, the username is username and the database server is localhost.
For more help, see the Installation and upgrading handbook. If you are unsure what these terms mean you should probably contact your hosting provider.