Well I am no expert, but I have thought of a couple of theories.
1) Hijacked, as said by the Malaysian PM, the plane stopped transporting its details. Its a common thing for hijackers to turn off the planes transponders to make it so air traffic controllers cannot be its height or speed.
You may argue that there is a locked door into the cockpit, but pilots do come out every so often.
2) This wouldn't be the first time but it is possible that military action was taken after the plane diverted off course.
As with Korean Air Lines Flight 007 and Korean Air Lines Flight 902.
3) Last is Pilot error and electrical or instrumental errors. I would like to remind everyone of how complex a aircraft of this size is
So in summary any number of things can go wrong and will go wrong.
