fatal error after 3.2 upgrade :(

Hi guys, after I clicked ‘upgrade automatically’ in the dashboard to 3.2, I get this:

Warning: require_once(/home/homename/webpage.com/wp-includes/class-json.php) [function.require-once]: failed to open stream: No such file or directory in /homename/webpage.com/wp-content/plugins/wp-to-twitter/functions.php on line 52

Fatal error: require_once() [function.require]: Failed opening required ‘/home/homename/webpage.com/wp-includes/class-json.php’ (include_path=’.:/usr/local/lib/php:/usr/local/php5/lib/pear’) in /home/homename/webpage.com/wp-content/plugins/wp-to-twitter/functions.php on line 52

What should I do….?