You will get this error if your website is not able to reach the database. This could either be due to a password authentication issue or the MySQL host might be down. What can you do? 


Open the wp-config file from the File Manager in cPanel. 


The portion you are looking for is this, 


/** MySQL database username */
define('DB_USER''username_here');
 
/** MySQL database password */
define('DB_PASSWORD''password_here');
 
/** MySQL hostname */
define('DB_HOST''localhost');


These details much match those you have in cPanel under the MySQL section. Reset the database username and password. On our cPanel servers your host is always 'localhost'.