FO DISTR WHEN AUTOPLT CLICKED OFF; CAUSING TURN AND DSCNT. PIC OUT OF COCKPIT. FLT ATTENDANT INJURY.

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WHILE CRUISING AT FL310; ATC REQUESTED A L TURN FOR TFC TO A HDG OF 310 DEGS (A 30 DEG L TURN). WHILE THE ACFT WAS TURNING; I LOOKED DOWN TO THE R TO PLACE MY CUP IN THE CUP HOLDER. I HEARD A CLICK. WHEN I LOOKED AT THE INST PANEL; THE AUTOPLT HAD DISENGAGED AND THE ACFT WAS IN A DSNDING L BANK. I IMMEDIATELY TOOK MANUAL CTL OF THE ACFT AND ROLLED THE WINGS LEVEL AND APPLIED BACK PRESSURE TO RETURN THE ACFT TO THE ASSIGNED ALT. I LEVELED OFF AND RE-ENGAGED THE AUTOPLT. A WHILE LATER; ORD FLT ATTENDANT RPTED SHE HAD FALLEN EXITING THE AFT LAVATORY AND HIT THE ARM REST ON AN AISLE SEAT. SHE SAID IT WAS A MINOR INJURY; BUT HURT. CAPT ADVISED HER TO WRITE IT UP AND CHK WITH MEDICAL UPON ARR AT ORD. SUPPLEMENTAL INFO FROM ACN 499792: IN CRUISE; I HAD JUST COME OUT OF THE FORWARD LAVATORY WHEN I FELT A 1-G PULL. I HELD ON UNTIL I COULD GET INTO THE COCKPIT. THE AUTOPLT HAD EVIDENTLY DISCONNECTED IN A TURN AND THE FO HAD RECOVERED. FLT ATTENDANT #4; ORD BASED; SUFFERED MINOR INJURIES. REFER TO FO ASAP FOR DETAILS.