When you see a message like this, it means that the database connection settings in your script's configuration file are not valid for some reason. The fix is to reset the password for the MySQL user in your Cpanel > MySQL Databases section, to the one used in your configuration files. You can find the names of the files with the DB connection settings of the most popular scripts below. If you encounter any problems, please, contact support.
WordPress wp-config.php
Joomla configuration.php
Drupal settings.php