Call to undefined function get_header() in /home/simplei6/public_html

When I was trying to insert Adsense code into my index.php file. My website started showing a Index/ page with a long list of links to my files. I uploaded my backup from earlier today and it now shows a page with

Fatal error: Call to undefined function get_header() in /home/simplei6/public_html/index.php on line 1

I then tried to upload a backup of the whole wp-content folder but it is still not working.

My website is simpleinfoonline.com