blank screen on checkout

I have set up my site to use cybersource as the payment gateway and fedex as the shipping provider. When I try to purchase a product, I can get all the way to the page that lets me review my order, but then when i click the "submit order" button, I just get a blank screen. My credit card seems to get charged by cybersource, but I just don't get any confirmation message or emails

After digging around in the code for a while, I realized that the uc_cart_checkout_review_form_submit function gets called, but that's where everything ends. The uc_cart_checkout_complete never seems to get called, and the url stays at cart/checkout/review