Just a couple of basic ideas..
First verify that your site is configured to serve index.php files, as well as index.html files. Your host will have information on how to make those changes if required.
If your site is already configured to serve index.php files, then verify that WordPress is actually installed in root, and not in a sub-directory in public_html
Example: if you installed WordPress in a directory named "wordpress" inside your public_html directory, then your install path would be yoursite.com/wordpress/wp-admin/install.php