I recently seen a problem with my products when adding to cart. It worked before, but now when you click buy (you see the products is added to the cart) but when I click go to checkout the products disappears and the cart is empty.
There are various options to check off which some are listed below:
1. The version you are using is old and it is essential to update it to the latest version.
2. You have used the secure ssl (https) on the cart page, to overcome this you have to either move the complete site to secure ssl or remove the ssl from the cart page.