I had a very strange issue with a client’s installation of magento a few nights ago. The checkout page was completely blank and the apache error logs were completely empty.
I tried all the usual fixes of looking at file permissions, updating the magento installation, checking for new modules, etc but none of these solutions solved the problem. Eventually, someone on the forums suggested to me that maybe I should see if Compilation was turned on. I completely forgot that I had turned on System Compilation to increase the performance of the site.
After, i went into ystem → Tools → Compilation and disabled it, the checkout page started working again and customers could continue to order.