There is one issue that returns a website being viewed a message that says “error establishing a database connection”. The case on one of the websites was that the owner upgrades his wordpress version in the the latest available through fantastico in the the control panel. The question was how did it just returned one website error while the other websites being upgraded worked fine? But one of the add-on websites? Somehow, somewhere there was a change made that returned an error of establishing database connection and therefore cannot view the website nor can login to wordpress though website/wp-login.php. The fix? Watch these awesome tutorial videos and we will learn a lot.
Others ran a query to their mysql database, or even upgraded their version which also worked for their particular problem. But for this error message, the answer lies on the videos above.