WordPress update 3.2 Site gone missing

I just updated my website with the 3.2 version via the dashboard and now it is showing below and my website is now gone missing. What has happened and how do I get it back up? I can’t even log into dashboard or see the site outside the wordpress on either of its .com or co.uk domains?????

Warning: require_once(/home/besoscon/public_html/wp-includes/class-json.php) [function.require-once]: failed to open stream: No such file or directory in /home/besoscon/public_html/wp-content/plugins/tweet-this/tweet-this.php on line 145

Fatal error: require_once() [function.require]: Failed opening required ‘/home/besoscon/public_html/wp-includes/class-json.php’ (include_path=’.:/usr/lib/php:/usr/local/lib/php’) in /home/besoscon/public_html/wp-content/plugins/tweet-this/tweet-this.php on line 145