Distracted by preparations for the approach and landing; the flight crew of a CE-750 failed to note the apparently uncommanded disconnect of the autopilot. Recovery followed a loss of some 1;800 FT and a heading deviation of 5 to 10 degrees.

Narrative

We were flying at FL430 about an hour from our destination. We began to review the arrival and approach briefing information. I felt the aircraft enter a turn which was normal as I was expecting our next waypoint. I then felt the bank angle continue to increase then observed the aircraft rolling through 30 degrees of bank; nose low; in a descent. I reached for the controls and autopilot disconnect at the same time the other pilot had already grabbed them and began to recover. I immediately contacted Chicago Center stating our autopilot malfunction and recovery and we returned to FL430. Once level the autopilot was reengaged and operated correctly. During the descent we lost approximately 1;800 FT and 10 degrees of heading.When the incident took place there were no audible warnings; such as autopilot disconnect or altitude alerter. Once we were level and the autopilot was reengaged we manually disconnected the autopilot and the aural warning worked. We reconnected the autopilot again for the remainder of the flight and monitored its performance very closely. After the flight we contacted Maintenance and made an information only write-up in the aircraft's discrepancy log.I don't think there was much that could have been done to prevent this from happening. Had I not been looking at the EFB we may have caught the deviation in a lesser time; but I am confident the aircraft would have still lost altitude. I think if the aural tones had worked; nothing would have come from the autopilot disconnecting.