Not exactly sure what happened, but this should get your site loading again. Download your index.php file and edit it and either remove all other code except for this below or replace the contents of your index.php file with this code below and upload the index.php file to your website.
<?php
/**
* Front to the WordPress application. This file doesn't do anything, but loads
* wp-blog-header.php which does and tells WordPress to load the theme.
*
* @package WordPress
*/
/**
* Tells WordPress to load the WordPress theme and output it.
*
* @var bool
*/
define('WP_USE_THEMES', true);
/** Loads the WordPress Environment and Template */
require( dirname( __FILE__ ) . '/wp-blog-header.php' );