Obviously, one of the variables set in config.php are wrong - either the database username or the database password - else your account has no access to the database you are trying to use.
If you are using cPanel, make sure you add that user to the Database. Creating a new DB user doesn't automatically add him to the database access.In your CPanel, click on MySQL. Also make sure (with cpanel) your database name INCLUDES your account name.
Like if you make a new db, and call it "vb". And lets say your Cpanel account name is "joeshmo".
Your database name is: joeshmo_vb
Same with username... (not password though!).
Hope now you can fix your bug .