Hi. My name is*****'m a camera repair technician with over 20 years in the business and I'll be glad to help you with your question today.
Due to the design of the 5D's it will need to be serviced by a Canon authorized shop. It sounds like the AF Sensor Module has gone bad, which is not unheard of in most all DSLR's. The camera will need to be re-programmed with proprietary software once the module is replaced. Here's a link to the Canon site to get you started on the repair process.