What would happen to the rudder on an A320 if both FAC systems failed? What would happen to the rudder of an A320 if both Flight Augmentation Computer (FAC) systems completely failed during mid-flight? Would the auto pilot immediately pass control back to the pilots or if they  failed would that be it? Would it be impossible for the cockpit to communicate with the mechanisms which control the rudder or is manual operation possible in an A320? 

Also, I know this maybe a more difficult/impossible questions but what would cause the malfunction of one of the FAC computers? Are there any likely scenarios or previous cases?
The A320 has a mechanical link from the rudder pedals to the hydraulic rudder actuators. Computer-generated rudder commands for yaw damping and turn coordination are added to the mechanical signal from the rudder pedals, but in the event of a two-sided computer failure the rudder can still be controlled.

The elevator trim wheels are also mechanically connected, so rudimentary control of the aircraft remains even in the event of a total electronic failure, as long as there's hydraulic power.