Fatal Error on Auto Update to 3.4

Hi there, I’m an absolute novice. When I saw the auto update I chose to perform it. It did not get past the installation screen. It stopped on something like ‘Installing Update…’ but never progressed. I deleted the .maintenance file via ftp and now have this fatal error.

Fatal error: require() [function.require]: Failed opening required ‘/home/xxxx/public_html/site/wp-includes/class-wp-theme.php’ (include_path=’.:/usr/lib/php:/usr/local/lib/php’) in /home/xxxx/public_html/site/wp-settings.php on line 115

I am petrified of losing months worth of content. I’ve tried reading through numerous guides and posts but I can not seem to find anything similar. I have no access to my admin panel. Is there anyone who could offer some advice? Thanks in advance.