Flighradar reports loss of height from 10,900 ft to 500ft in under a minute before it disappeared. That's a nosedive!
Something catastrophic must have happened, but at least for the passengers and crew it was quick. God rest their souls.
For Boeing it is really bad news. It was a 737-500 and not a 737-MAX, but it was of the same family and that will make passengers nervous. The MAX has only just been certified to fly again and you can bet good money that questions will be being asked.
Indonesia is politically stable so it is highly unlikely that it was some sort of terrorist attack and that leaves technical or pilot failure. The Java sea off the coast is shallow so the black boxes should be recovered fairly soon.