Why is my WordPress site giving me an "Error establishing a database connection" error?

Question

When navigating to my WordPress website, why is it giving me an "Error establishing a database connection" error?
 

Answer

The first step would be to enable debug logging on your WordPress installation by changing the following in your wp-config.php file in your WordPress installation directory:

define('WP_DEBUG', false);

to be "true" like below:

define('WP_DEBUG', true);

This will expose a new error on the top of your WordPress site that will reveal the cause of the database connection issue. 

One common error you might see is:

Warning: mysqli_real_connect(): (HY000/1045): Access denied for user 'USER'@'localhost' (using password: YES) in /home/user/public_html/wp-includes/wp-db.php on line 1626
Access denied for user 'user'@'localhost' (using password: YES)

which is generally caused by the username and password for your WordPress database not being accurate. You should double-check your credentials and make sure the details in your wp-config.php match what the credentials are.