It should be noted that in Aviation, there are two types of radar, primary and secondary.
Primary radar is when radio waves are sent out and bounce off the skin of an aircraft and return to the sending station. This system has a finite range and can only show the relative position from the station and a relative bearing on where it's going.
Secondary radar is a system that uses transponders installed in aircraft that specifically reply encoded signals back to a ground station. These signals give specific data to ATC such as the aircraft's identification, speed and altitude.
When flights are out over water, they may only be seen through secondary radar, so if a transponder is turned off or power is lost or it fails, then ATC loses them on the screen.
I do not believe this to be another case like MH370, but instead possibly an in-flight break up. If this is true, wreckage should be found within the next few days.