Probably a stupid question, but is it possible that the communications/radio only on the pilots end had some kind of fault?
Like they were trying to call for help but for whatever reason ATC didn't recieve it...
Which could be why ATC never heard from the plane during what we presume would be a mayday situation.
Another possible thing is could both pilots have fell asleep while the plane was on autopilot and something happened.
Possible that both fell asleep but if anything went wrong they would've woken up thanks to all the alarms, but if the plane did turn around as suggested then they def weren't asleep
Lots of smart people in the industry are saying transponders just don't go off, so no doubt something has happened to the plane
Potential causes are terrorist
Bomb
Structural failure/disintegration
Cockpit fire
Something else which is probably most likely
Problem is each different cause could put the plane in a different location