New update crashes WP

I just successfully (so I thought) updated to WP 3.2. The update has completely crashed my site. The site is down and I cannot even get to the dashboard. I assume the problem is with the plug-in TWEET THIS, but because I cannot access the dashboard, I cannot remove the plug-in. Ideas? Here is the full text of the error message I get:

Warning: require_once(/home2/dailypr1/public_html/wp-includes/class-json.php) [function.require-once]: failed to open stream: No such file or directory in /home2/dailypr1/public_html/wp-content/plugins/tweet-this/tweet-this.php on line 145

Fatal error: require_once() [function.require]: Failed opening required ‘/home2/dailypr1/public_html/wp-includes/class-json.php’ (include_path=’.:/usr/lib64/php:/usr/lib/php’) in /home2/dailypr1/public_html/wp-content/plugins/tweet-this/tweet-this.php on line 145