Wordpress "unable to establish a database connection"

Have you just uploaded a new version of Wordpress via ftp?
Did you overwrite wp-config.php ?

You now have two options: restore the file from the backup you made before the upgrade.
You did make a backup, right?

Alternatively, use the File Manager in your control panel (CPANEL, PLESK, etc), to edit the wp-config.php file.
(It'll be in the root of your Wordpress install)

You need the following information:

Database Name
Database User Name
Database User Password
Database Host Name. (Usually "localhost", but NOT always. It's not on 000webhost for instance)

On Cpanel, all these values can be read from either "mysql" or "phpmyadmin", apart from the password. You may need to set a new password. On Plesk it's under databases - database users. Again, you may need to set a new password.

Now enter these values in the correct places in wp-config.php and save the file.

Next time, take a backup.

 and that's it.