Well, you need to open wp-config.php by transferring it to your computer via FTP and then opening it in your text editor. You're looking for the following stuff:
// ** MySQL settings - You can get this info from your web host ** //
/** The name of the database for WordPress */
/** MySQL database username */
/** MySQL database password */
/** MySQL hostname */
/** Database Charset to use in creating database tables. */
/** The Database Collate type. Don't change this if in doubt. */
It's all in the top section of the file.
Basically, if any of the data in the MySQl settings is different to what your ISP gave you when you set up the database, then WordPress can't work. You mentioned that you have other blogs working in the same hosting account. How many databases does your ISP allow you to have?