This error message generally means that your Joomla's configuration.php file has either the wrong database name or database username. In our testing environment, we changed both the database username and password, and in each separate occasion the same error message occurred.
You should review your configuration.php file and ensure you are using the correct MySQL username and password.
These settings will appear in the file as:
public $user = 'userna5_dbuser';
public $password = 'dbpass';
If you are sure the settings are correct, it is recommend to reset your database user's password and update your configuration.php file with the new password. This will help ensure that you have the correct database password set.