I got this error in Wordpress.

*/wp-includes/functions.php) [function.require]: failed to open stream: No such file or directory in  */wp-settings.php on line 65
and blank page.

I solved it by looking on page: http://mydomain.com/readme.html
it told me my Wordpress version number (was 3.6.1). Then i went to: http://wordpress.org/download/release-archive/ and downloaded 3.6.1 zip file.

Extracted original Wordpress ZIP file and zipped /wordpress/wp-includes folder. Uploaded this zip into my wordpress main folder. Backed up original wp-includes and extracted the one i just uploaded. So files in wp-includes got overwritten and added possibly missing files. And wordpress started working.