Error displaying the error page: Application Instantiation Error in Joomla 3

Error page :

Error displaying the error page: Application Instantiation Error

Problem:
The following issues caused this error message to appear:

• The database username in our configuration.php file does not match that of what is configured on our hosting account
• The database password in our configuration.php file does not match what of what is configured on our hosting account
• The database hostname in our configuration.php file could not be connected to

Solution:
In your Joomla configuration.php file, ensure that your database username, password, and hostname match what is set / provided by your host. They will be defined in the following variables in your configuration.php file:

// database username goes here
public $user = ‘database-username’;
// database password goes here
public $password = ‘database-password’;
// database server name
public $host = ‘localhost’;