B737-500 FLT CREW LOSES BOTH AUTOPLTS IN RVSM AIRSPACE.

Narrative

EXPERIENCED A DUAL AUTOPLT FAILURE. WE WERE AT FL370 WITH AUTOPLT B ENGAGED IN LNAV AND VNAV. THE FO LEFT THE COCKPIT TO USE THE RESTROOM. THE AUTOPLT DISCONNECT HORN SOUNDED SO I TOOK MANUAL CTL OF THE ACFT AND PUSHED THE B AUTOPLT BUTTON IN AN ATTEMPT TO GET THE AUTOPLT BACK ON LINE. THE B AUTOPLT WOULD NOT REENGAGE. I WAS HAND FLYING THE ACFT AND REACHED OVER AND PUSHED THE A AUTOPLT BUTTON; THE A AUTOPLT WOULD NOT ENGAGE EITHER. THE FO RETURNED AND I CONTINUED FLYING THE ACFT. I DIRECTED THE FO TO ATTEMPT TO REENGAGE THE AUTOPLTS AFTER CYCLING THE MASTER AUTOPLT DISCONNECT SWITCH. NEITHER AUTOPLT WOULD ENGAGE. I THEN HAD THE FO CALL DISPATCH AND GET A PHONE PATCH TO MAINT CTL TO TROUBLESHOOT THE PROB. I WAS FLYING THE ACFT AND WORKING THE RADIOS WITH ATC. I INFORMED ATC OF OUR SITUATION AS WE WERE IN RVSM AIRSPACE WITH NO AUTOPLT. THE FO WITH DIRECTION FROM MAINT CTL CYCLED THE CIRCUIT BREAKERS FOR BOTH AUTOPLTS WITH NO LUCK. I INFORMED ATC THAT WE WERE UNABLE RVSM AND THEY HAD US DSND TO FL270.