When we upgrade the wordpress from 3.5.1 to latest some time upgrade process is not done successfully and showing blank screen. When we active the debug mode in wp-config.php
wordpress file then showing the error as below.
Fatal error: Call to undefined function get_post_format_slugs() in /public_html/wp-includes/theme.php on line 1382
Sometimes this problem occurs due to older version theme or plugins. When we move all plugins from plugin directory then still shown the same problem. So we should activate the default theme before upgrading the wordpress. In this way wordpress upgrade process will complete successfully after this we can activate the theme.