Given the altitude and airspeed at the point of disappearence the plane, or parts of it, will likely be quite some distance south east of the last know location, which could be in shallower parts of the Med.
Discarding the usual creative but pointless theories, all we know with any degree of certainty is that the plane disappeared suddenly at cruising altitude (or thereabouts) having just entered Egyptian airspace. The rapidity of the event suggests some kind of very sudden 'mechanical' failure - cause unknown. Realistically there are only two broad possibilities; explosive device or a sudden mechanical/structural/engine-type failure.
For some reason I can't help feeling that there is some significance in whatever happened did so just as the flight entered Egyptian airspace.