Marketpress plugin not working and blank screen wp-admin

Marketpress plugin doesn't work. When I add a product to cart and click on checkout I get a message "There are no items in your cart."

Here's the website
http://newearthcentral.com

I cannot access http://newearthcentral.com/wp-admin
When I go to that page I get a blank screen.

It was working fine till yesterday but all of a sudden it's not working anymore.