If your wordpress site was working fine and out of nowhere you started seeing the following error:
Error establishing a database connection
Then this is the best and easiest way to fix it:
Add the following line in your wp-config.php file. Add it just before ‘That’s all, stop editing! Happy blogging’ line wp-config.php.
Once you have done that, you can see the settings by visiting this page: http://www.yoursite.com/wp-admin/maint/repair.php
Remember, the user does not need to be logged in to access this functionality when this define is set. This is because its main intent is to repair a corrupted database, Users can often not login when the database is corrupt. So once you are done repairing and optimizing your database, make sure to remove this from your wp-config.php.