Not the first time...
I recall the Boeing 737-400 Adam Air Flight 574 accident - 1st Jan 2007 - departed cruise altitude of FL350 and disappeared into the Straits of Makassar. Nothing was found until 10 days later when fisherman came across a large fragment of the tail. The black boxes were located by a US Navy Ship, the Mary Sears, 27 days after the disappearance, and took considerably longer to recover them.
Probable cause of the Adam Air accident was loss of control whilst attempting to trouble shoot faulty INS system.