Just watched a ten minutes news story from a couple of months back about this trip. It’s not unusual for things to go wrong with communications so HOPEFULLY it’s just a communciations glitch.
If it’s just a communication glitch, then why didn’t they come back up when they were supposed to?
The descent and return was to take about 8 hours, so it would have been back up by now. I would imagine ships and planes are combing the nearby surface in case that’s what happened.