I know very little but after teaching myself and working with two different hosting providers and several sites over the past year or so, I DO know this:
Your site does not KNOW how to display itself without the index.php - that is the root file that tells it how to display, which files to read in what order, etc. Do you have a backup copy of it that you can reinstall? Can you get one from your host? Try re-installing that asap. If after doing that you are still getting nothing to show on your domain, go to where you downloaded your wordpress files locally. Find the index.php file there (under wp-admin, wp-includes, etc - it is a single file) and FTP that over to your site - overwrite it on your public html file.
That should get your wordpress site to show. Good luck and let us know how it goes!