Wordpress: Error establishing a database connection

A number of things can cause this error.

1) Misconfiguration

First to go to http://<wordpress site>/wp-admin

Wp-admin code will give you some hints to see whether the issue is with username and password or the db_name.

For my case, the db_name in wp-config.php was not exactly the same as the db_name I had in mysql. So it was a quick fix.

2) Low disk space

I recently ran into this situation. Mysql would simply fail to start.

Definitely check free space available when you are troubleshooting this message. Rule this one out quickly.