Fire
I got curious and did a research on the antenna locations on the B-777. As I imagined, all the comm related antennas are under the "belly" and directly under the forward cargo bay. All the sat comm antennas are, of course, on the top of the fuselage.
Could it be possible that a fire in the front cargo bay destroyed the wiring "traveling" to the lower antennas? The upper antennas continued sending signals to Inmarsat and dense and toxic fumes took over the cabin and cockpit.
If dense smoke takes over the cockpit, it is impossible to fly any airplane. Change course on the FMS with almost no visibility very difficult, low fire but lots of smoke. For how long a crew can resist?
Antennas out?