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. Official cPanel documenation explaining how to change the password for a MySQL user can be found here. You can find the names of the files with the database 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
If you use Softaculous for your scripts installation, you can find all the necessary database infromation there.You will also find instructions on how to see details of your installations without looking for configuration files here.