How about a massive electrical failure caused by electromagnetic radiation from an external source. Electrical systems rendered inoperative. Engines continue to run as fadecs shielded and supplied by permanent magnet alternators. FBW Flight controls out but aircraft could be turned with rudder deflection, is that physically connected to the pedals on the 777? But with no way to reduce thrust and cabin alt rising crew would then have had to don masks hampering communication, or may have become hypoxic. So in darkness with no roll or pitch control, no way to reduce thrust, and possibly impaired judgement perhaps they could have ended up in the water south west of Australia 7 hours later when the fuel ran out.