I assume one of three things happened- they somehow got snagged on something coming and can’t pull free. (This makes little sense, since… any competent engineer wouldn’t have stuff to get snagged in the first place and there’s not much down there to get snagged on, except the wreck itself.)

They could have suffered a leak in the buoyancy tanks preventing their return, this seems a bit more likely.

Or they had a failure in the electronics- a short in the electrical system could leave them without power.

The banging rules out the fourth- a leak in the pressure vessel (which at depth they wouldn’t live long enough to know.)