template-loader line 26 in 3.4

Hello,

I received a similar error as this fellow, but did so during a manual upgrade:

Fatal error: Call to undefined function get_front_page_template() in /usr/www/users/cvarley/wp-includes/template-loader.php on line 26

When trying to do anything in the dashboard, I’m prompted to upgrade my database, but clicking to do so automatically gives me the following error:

Fatal error: Class ‘WP_Theme’ not found in /usr/www/users/cvarley/wp-includes/theme.php on line 106

Perhaps relatedly, I’ve found that any attempt to do an automatic upgrade of any kind on this particular site gives me a White Screen of Death.

Any help would be much appreciated.