There’s a common saying that you can’t die from holding your breath, because once you pass out, the autonomic nervous system takes over and you’ll start breathing again.
The inspiration for this question is this video of a guy who apparently almost died from holding his breath too long, and being told to breathe while he was blacked out is what saved his life. People in the comments are praising the rescuers for saving the guy’s life by slapping him and shouting at him to breathe after he passed out from holding his breath, and are acting like had the rescuers not yelled at him to breathe, he surely would’ve died.
The video is showing something called a freediving blackout, it’s a condition that gets triggered in divers such as the one shown, plus other factors such as hyperventilating prior to the dive.
What happens in this case is the person loses consciousness because they don’t feel the need to breathe as they have purged a lot of the CO2 the body has prior, giving them “room” to not feel the urge to breathe.
Again, as specified previously, assuming the person doesn’t die of anything immediate in the water, the body simply stops breathing.
The problem is when it starts again.
Your body, once “jolted awake” by the involuntary nervous system, will try to breathe, and it doesn’t know or care that you might still be underwater since you stopped swimming and didn’t get out of the water in time or weren’t rescued or washed ashore on some deserted island.
If you’re out of the water at this point, you’re fine, usually. If you’re still underwater, you breathe in (involuntarily) a bunch of the water (or a bunch of “nothing” if you got your holes plugged) and that’s what kills you.
Kind of ironic that in an attempt to preserve your life, your own involuntary bodily function is what ends up being the final nail.