Best guess is that its suffered a catastrophic power outage, no idea how it will have plummet to the ocean bed without controls, it'll have been engineered to survive much more pressure than expected to endure (basic engineering percentages) but is life support operational automatic or were they burning oxygen a matter of hours into the operation. They could well be sat in pitch black and cold, with a couple of sandwiches and a bottle of water between em. Sub could be on its side or upside down, no idea what fail safes for comms are built in, auto sos buoy? A very grim situation to face. Apparently there's at least one sub lost with all lives (three) previously?