Coursespress/Marketpress completely wonked after update

After a recent update, all the courses were set to $0 on the products page, even after the database update. I fixed that by changing the product price, but now, you can't buy anything - I get a blank screen after registration of a new user, and the cart says I have 5 things (I only chose one course,) but I can't even access the cart - it redirects me to the course listings page.

Help!