Description: This error happens when your database password was modified and you haven't updated your PHP scripts with the new value (generally happen at WordPress on the wp-config.php). It could also be that your database server is down and you need to contact your hosting company to get it fixed.
This means that your site is down and it should be fixed asap (availability issue).
Error establishing a database connection
Clean up: Update the password on the wp-config.php or contact your hosting company. If you have any question, contact support for help at firstname.lastname@example.org.