does the page load if you run it as index.php?
if so, remove index.shtml and put the following in your .htaccess
Code:
RewriteEngine on
RewriteRule ^index.shtml$ index.php [L,QSA]
I'm assuming you have tons of links pointed at index.shtml and can't use a new page, but, the above will probably fix it without having to involve your host -- and if you are in a virtual environment, they may not be willing to enable the permissions to allow those directives, or, its possible that you're running php under fastcgi and the directive isn't quite doing what you need.