Ciao Star, nella root (directory principale) del tuo sito c'è il file : configuration.php

Eccone il contenuto, se cambia un pò non farci caso:

<?php
/**
* @version $Id: configuration.php-dist 11409 2009-01-10 02:27:08Z willebil $
* @package Joomla
* @copyright Copyright (C) 2005 - 2008 Open Source Matters. All rights reserved.
* @license GNU/GPL, see LICENSE.php
* Joomla! is free software and parts of it may contain or be derived from the
* GNU General Public License or other free or open source software licenses.
* See COPYRIGHT.php for copyright notices and details.
*
* -------------------------------------------------------------------------
* THIS SHOULD ONLY BE USED AS A LAST RESORT WHEN THE WEB INSTALLER FAILS
*
* If you are installing Joomla! manually i.e. not using the web browser installer
* then rename this file to configuration.php e.g.
*
* UNIX -> mv configuration.php-dist configuration.php
* Windows -> rename configuration.php-dist configuration.php
*
* Now edit this file and configure the parameters for your site and
* database.
*/
class JConfig {
/**
* -------------------------------------------------------------------------
* Site configuration section
* -------------------------------------------------------------------------
*/
/* Site Settings */
var $offline = '0';
var $offline_message = 'This site is down for maintenance.<br /> Please check back again soon.';
var $sitename = 'Joomla!'; // Name of Joomla site
var $editor = 'tinymce';
var $list_limit = '20';
var $legacy = '0';

/**
* -------------------------------------------------------------------------
* Database configuration section
* -------------------------------------------------------------------------
*/
/* Database Settings */
var $dbtype = 'mysql'; // Normally mysql
var $host = 'localhost'; // This is normally set to localhost
var $user = ''; // MySQL username
var $password = ''; // MySQL password
var $db = ''; // MySQL database name
var $dbprefix = 'jos_'; // Do not change unless you need to!

/* Server Settings */
var $secret = 'FBVtggIk5lAzEU9H'; //Change this to something more secure
var $gzip = '0';
var $error_reporting = '-1';
var $helpurl = 'http://help.joomla.org';
var $xmlrpc_server = '1';
var $ftp_host = '';
var $ftp_port = '';
var $ftp_user = '';
var $ftp_pass = '';
var $ftp_root = '';
var $ftp_enable = '';
var $tmp_path = '/tmp';
var $log_path = '/var/logs';
var $offset = '0';
var $live_site = ''; // Optional, Full url to Joomla install.
var $force_ssl = 0; //Force areas of the site to be SSL ONLY. 0 = None, 1 = Administrator, 2 = Both Site and Administrator

/* Session settings */
var $lifetime = '15'; // Session time
var $session_handler = 'database';

/* Mail Settings */
var $mailer = 'mail';
var $mailfrom = '';
var $fromname = '';
var $sendmail = '/usr/sbin/sendmail';
var $smtpauth = '0';
var $smtpuser = '';
var $smtppass = '';
var $smtphost = 'localhost';

/* Cache Settings */
var $caching = '0';
var $cachetime = '15';
var $cache_handler = 'file';

/* Debug Settings */
var $debug = '0';
var $debug_db = '0';
var $debug_lang = '0';

/* Meta Settings */
var $MetaDesc = 'Joomla! - the dynamic portal engine and content management system';
var $MetaKeys = 'joomla, Joomla';
var $MetaTitle = '1';
var $MetaAuthor = '1';

/* SEO Settings */
var $sef = '0';
var $sef_rewrite = '0';
var $sef_suffix = '';

/* Feed Settings */
var $feed_limit = 10;
}
?>

Se hai cambiato dei dati inerenti al Database devi aggiornarli nelle parte che ti ho evidenziato in grassetto.
Puoi modificarlo online, tramite un qualsiasi client FTP oppure ti scarichi il file sul PC, lo modifichi e lo ricarichi di nuovo sul sito, tutto qui.

Grazie Jarod