Some people here have suggested loss of control due to a stall. But surely, even at night time, with possibly no air speed info on the primary display, a stall (or even a spin) is recoverable from 35,000 feet.
A second point... given that this was on an IFR route, surely the wreckage can be found by flying the same route at 2000 ft, at 100kts?
If this was a squawk 7500 situation, surely secondary radar would have picked up the deviated flight path, even if all onboard comms were disabled?