Fix an issue with ecommerce website

There seems to be an issue with my website. To replicate it please visit this page: [login to view URL] Add the computer to your car by clicking buy. Click checkout and watch carefully. The form loads for guests, where they need to enter their address and other information, but then once loaded, it disappears. I need this ASAP and FIXED that means NO WORKAROUNDS. You need to explain what was going wrong and how you fixed it. Please work out what the issue is before bidding. No automated companies with copied text pm's will be chosen.