Shopping cart, products suddenly failed.

Without making any changes to the site, I suddenly noticed that my shopping cart (powered by the thecartpress plugin) failed with an error message (see the failed widget with error code here)
On the plugin community support they suggested updated the plugin. I updated the plugin,
and now the shopping cart does work on the homepage at first,
BUT when i click on any of the products page, the shopping cart fails for that page and then the homepage.
ALSO the products tab on the dashboard has disappeared, along with the products.
HELP!!!
what should I do?!?!?!

Also, here is the error message:
Fatal error: tcp_get_shopping_cart_detail() [function.tcp-get-shopping-cart-detail]: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "ShoppingCart" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /hermes/bosweb/web166/b1660/ipg.culturecrossorg1/test/wp-content/plugins/thecartpress/templates/tcp_general_template.php on line 159