There's a few possible scenarios here. This submersible had lost comms before on other dives and eventually regained communication with the mother ship. Most likely what happened here but then didn't regain comms.
In case of electrical failure, the sub has some ballast weights it would drop which would make it float to the top.
It could be somewhere in the ocean on or near the surface, which I think would be really hard to see because it was a dumb idea to paint it white and blue. This case it's only a matter of time before they run out of air vs someone finding it and seeing it in the vast ocean.
It could have gotten stuck in the titanic wreckage on a cable or something on the sea bed..worst outcome since they dont have another sub to go that deep and even navy subs don't go that far down.
Or it had some sort of electrical failure, floated up again as a safety measure but reached neutral buoyancy and is not at the surface but floating somewhere in the middle of the ocean, also horrible outcome as it would be extremely hard to find.
It sounds morbid but best case scenario would be the port hole glass which had reports of not even being manufactured to be in water greater than 1300 meters, had a failure since it has been in 3 other dives and wear and tear took place..and because of the pressure the sub imploded..which would mean passengers died before they even knew what was happening.
I dunno but if I'm a millionaire and I pay $250k to go on this expedition, and you show me this vessel upon arriving, that's controlled with a PC controller from 1998 or whatever, I'm turning around and going back to my millionaire lifestyle, no fuss.