I was on 3.5.2 and did upgrade to 3.6 when I got the following message:
Fatal error: Call to undefined function wp_initial_constants() in /home/nepa2z/public_html/samudayak/wp-settings.php on line 26
Now i can't even login to wp-admin or any page. This happened when i upgraded from wp-admin.
you can view the error in http://www.nepala2z.com/samudayak/