There were no distress calls that we know of, which would imply that something sudden and catastrophic took place that gave the crew no time to put out a distress call. This is 2016 - planes don't just fall out of the sky.
https://en.wikipedia.org/wiki/Japan_Airlines_Flight_123
https://en.wikipedia.org/wiki/Contin...ss_Flight_2574
https://en.wikipedia.org/wiki/China_Airlines_Flight_611
https://en.wikipedia.org/wiki/Indone...ia_Flight_8501
.. plenty of other explanations, sadly.