This has happened a couple of times for me. Last night, I was cruising on autopilot when suddenly, the AP cut off, and the plane began to dive. After hand-flying for a while, I realized that the Pitch Trim and Yaw Damper switches had been turned off. Once I switched them back on, I could re-engage AP and finish the flight. Until I figured that out, I could not turn on FD 1 or engage AP.
I'll try to gather more info if it happens again.