Can't Acess Dashboard After Upgrade to 3.2

I upgraded/updated my WP installation this morning and now can not access my blog or my blog dashboard. Would appreciate some advice/help.

Here is what is displayed when I try:

Warning: require_once(/home7/cyberbre/public_html/wp-includes/class-json.php) [function.require-once]: failed to open stream: No such file or directory in /home7/cyberbre/public_html/wp-content/plugins/tweet-this/tweet-this.php on line 145

Fatal error: require_once() [function.require]: Failed opening required ‘/home7/cyberbre/public_html/wp-includes/class-json.php’ (include_path=’.:/usr/lib/php:/usr/local/lib/php’) in /home7/cyberbre/public_html/wp-content/plugins/tweet-this/tweet-this.php on line 145