How do you get lost underwater? Just head up.
If it's the same sub that Cameron made his documentary series with, it was tethered to a boat too...
Both of these are actually surprisingly fallible, and it is worth remembering that exploration at depths like this is only
slightly less dangerous than space travel, mostly because you don't use a rocket to get there which can explode.
Part of how we know where to go is reference points, and in open blue water there's a pretty serious phenomena which mostly impacts scuba divers which is getting disoriented because there are
no reference points, just blue in every direction. This is compounded at night where the potential reference point of "it's lighter up there" is also gone. I say "potential" because it's very possible to get lost at depth even with light penetrating.
Down at the depths they're going to, there is no light anymore, and there is also basically no reference points until you hit bottom...but the abyssal plain is almost like a desert of silt usually, and you still have shit visibility due to the no light problem. Oh, and another fun detail is that anything disturbing the silt such as thrusters can cause a cloud of impenetrable murk that can last for a long time down there even with light to see with...
So you lose your navigation systems and lights due to a power failure and a redundant system also fails...you can be in a world of shit down there really fast without a breach.
The tether is also surprisingly fallible: strong underwater currents as well as the weak points such as where that tether attaches have all kinds of points of failure due to the deceptively strong force currents exert on something that long. The other issue is having enough cable strong enough to withstand all the shit to go down that deep.
Now, assuming a perfectly functioning emergency system to return bouyancy to the sub, that could take the issue of reference points out of the equation...but what if it's not perfect and there's a problem there and no way to fix it? If something external gets jammed (a nontrivial issue at those pressures) and that is required to resurface, you're fucked. Let's assume for the sake of argument NOTHING goes wrong and the sub pops up...but well away from where they expect to due to those currents I mentioned earlier, navigation is down, and so are communications.
Now you need to find a tiny dot in an often turbulent sea, and at this point they don't know if they should be looking on the surface
or down below. They have a fighting chance on the surface at least, but ocean currents can take things places nobody expects with a quickness.
