If you mean the one in your installation directory, you can just download WordPress .zip file again and replace it with the one from there. If you mean the index.php located in your theme file, I recommend you restore it from the original or a backup copy of the theme. (although I’m guessing by your request that you have no backup files). Or, completely clear the contents of the index.php file and paste this back into it.
<?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('./wp-blog-header.php');
?>
Leave no spaces at the top of the page. And start doing some reading on how to backup your site.
I will assume two things. One, you are using WordPress 2.7.1. Two, you mean the index.php located in your WordPress directory. Make changes at your own risk.
