The cart selections that were loaded for checkout have expired. This might have happened if you returned to the page after some time of inactivity. But don't worry, your selections should still be available from the non-checkout part of the site.
To prevent issues these items were NOT automatically re-loaded for checkout. Instead, click the button below to return to the homepage and then click on the cart button in the top menu to get access to the Checkout button to start the checkout process again.