1. Download a Fresh Wordpress Install - Make sure the version is the same as the one already on your server.
2. Unzip and find the index.php file - Replace the one you edited with this new index.php
3. Goto yoursite.com/yourwordpressfolder/wp-admin (replace yourwordpressfolder with the name of the directory you have your install in) and login
4. Goto your general settings and change back your Blog URL to the actual directory listing. For example, if your blog lies in sarah.com/myblog enter that into the second field which reads "Blog URL"
5. Post back here with how it went.