Are you seeing this screen when visiting your WordPress site?
This error indicates that WordPress in unable to connect to the database which contains the Pages, Posts and other essential information required to run your website, specifically the main causes are:
• The database server is not running
• The database server is running but database has been removed.
• The database server is running, the database is ok but the username or password has changed
These details are configured in the file wp-config.php that can be found in the root directory of your WordPress install. You should not change this file if you are not confident doing so as this can further break your installation of WordPress.