What is that username?
Could you try creating a simple PHP file (eg. dbconnect.php) in public_html on your heliohost website with the following code and navigate to it (eg. http://yoursite.heliohost.org/dbconnect.php) in your browser?
<?php
$server = "localhost";
$username = "FILLIN";
$password = "FILLIN";
$db_name = "FILLIN";
mysql_connect($server,$username,$password) or DIE("Couldn't connect locally.");
mysql_select_db($db_name) or DIE("Couldn't select database.");
echo "Database connection successful.
?>Replacing FILLIN with your details.
Unfortunately that error message isn't very specific. Is there any way you could get more details on the error? Could you also try what I have suggested above?