This is a very generic message, are you using IE? Try using FireFox instead, this may show the real error that is occurring.
If not, try adding the following at line 1 of the contactus.php page:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
?>
if this still does not show the error that is occurring, you will need to contact your hosting company to ask them to look into the error logs. Once we know the actual error we can begin to trouble shoot the problem.