Its ok loua you're frustrated, we understand and we really just want you to understand so its easier on you despite some of our comments always take them with a grain of salt my friend

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- Think of it this way, yes you're right its stored right there in the file but how can they get to it using my example above? If anyone could simply download that file hackers would be taking down sites by the second, most software vBulletin, IPB, even free phpBB forums, Wordpress, the lot of them all use some form of configuration file where the details are stored.
Regarding your issue: Yes, if you went into cpanel and changed the database users password, then nothing "automatically" changed it everywhere else for you

so with that being said hurry and edit config.php with the new password and it should come right back up

. Also you cannot simply rename config.php to another name unless you make other file edits, best to leave it as-is unless testing as Cell mentioned above. One other thing to mention is, whomever setup the forum initially had to manually rename config.php.new to config.php, then edit the file and enter in your database name, username, and password to the database so that is why most of us were shocked by your statements - we just couldn't figure out why this was just now surprising you... I see where you were coming from, sure its thinkable but glad we steered you in the right direction!