Recently i did a aptitude dist-upgrade on debian squeeze lts. After that my wordpress-site vanished, i only got a empty page.
Looking into /var/log/apache/errors.log shows:
[Thu Sep 25 22:44:47 2014] [error] [client 192.168.1.1] PHP Fatal error: Call to undefined function wp_get_current_user() in /usr/share/wordpress/wp-includes/capabilities.php on line 1281
This can be solved by putting
in the second line, after the php tag.
Then the site should be back.