Fatal Error

I’d been editing the same article several times. I returned to this article after noticing a typo, but when I tried to edit it I got the following error message-

Fatal error: require() [function.require]: Failed opening required ‘…/blog/wp-includes/Text/Diff/Renderer.php’ (include_path=’.:/usr/lib/php:/usr/local/lib/php’) in /home/a5655787/public_html/blog/wp-includes/wp-diff.php on line 5

I haven’t changed anything today, other than chmodding the directory my theme is in to 777 and changing which categories display on the front page (only had to change the cat id).