Upgrade to 3.4 from 3.3.2 failed

Hi please can you help,

I just backed up my wordpress database and updated my plugins with available updates before clicking the upgrade button.

On clicking update wordpress I was informed that the update had failed due to the following,

Warning: require(/home/prepayas/public_html/blog/wp-includes/option.php) [function.require]: failed to open stream: No such file or directory in /home/prepayas/public_html/blog/wp-includes/functions.php on line 8

Fatal error: require() [function.require]: Failed opening required ‘/home/prepayas/public_html/blog/wp-includes/option.php’ (include_path=’.:/usr/lib/php:/usr/local/lib/php’) in /home/prepayas/public_html/blog/wp-includes/functions.php on line 8

The above is also what I see when I try to login to the blogs control panel.

I have no idea where to begin with this, so any help you can provide would be great.