What happens to one in a locked in syndrome?

Death, in the realm of humans and Devas (for them), where Dhamma can be understood, is defined with "break up of the body", so a "unconscious" human being should not be regarded as no more living as long the body "works", ages.

22. "And what is aging and death, what is the origin of aging and death, what is the cessation of aging and death, what is the way leading to the cessation of aging and death? The aging of beings in the various orders of beings, their old age, brokenness of teeth, grayness of hair, wrinkling of skin, decline of life, weakness of faculties — this is called aging. The passing of beings out of the various orders of beings, their passing away, dissolution, disappearance, dying, completion of time, dissolution of the aggregates, laying down of the body — this is called death. So this aging and this death are what is called aging and death. With the arising of birth there is the arising of aging and death. With the cessation of birth there is the cessation of aging and death. The way leading to the cessation of aging and death is just this Noble Eightfold Path; that is, right view... right concentration.

23. "When a noble disciple has thus understood aging and death, the origin of aging and death, the cessation of aging and death, and the way leading to the cessation of aging and death... he here and now makes an end of suffering. In that way too a noble disciple is one of right view... and has arrived at this true Dhamma."

There is the state of (trans.asaññasatta) "Unconscious" - being, a realm of existence not desired and maybe therfore not much talk about.

How ever, even if "really" in an asañña state (lit. "No rememberance, no perception"), there are still the mental factors, consiting with form/object a living being (satta), of cause very subtle _{[Sadhu for giving the right spelling of this word, N. Chris]}.

So this asañña state might fit to you question.

There is also, what is called, "caught in concentration" in meditation, one might know such "unconscious" states, often called "daydreaming", when the mind is firm on a mind object concentrated (possible the arupa-realms of existence.

One in such a state is not easy to get to mind again, like "paralyzed", similar maybe with shock.

It's reported that Samadhi sessions can last for extreme long time, of course it has it's limits in regard of physical nurishment, but obiviously, the mind can nurish it self, for a long time, without much external imput. (see the four kinds of nurishment).

It's not so easy to say if this states might be in the sphere of asañña or actually arupa states, which might also explain the attentive discussion under the question, assuming two different phenomenas.

Of cause, in a very materialistic world of thoughts, the "mindless" states of beings will increase, either as general "ignorance" or such as the "medical" recognised phenomenas.

A modern wrong view, present in that what is called "secular Buddhism", is the case, the Buddha told, "when a asaññasatta falls out of its existence and gains a human life, this and that "materialistic" views are hold.

"There are, bhikkhus, certain gods called 'non-percipient beings.' When perception arises in them, those gods pass away from that plane. Now, bhikkhus, this comes to pass, that a certain being, after passing away from that plane, takes rebirth in this world. Having come to this world, he goes forth from home to homelessness. When he has gone forth, by means of ardor, endeavor, application, diligence, and right reflection, he attains to such a degree of mental concentration that with his mind thus concentrated he recollects the arising of perception, but nothing previous to that. He speaks thus: 'The self and the world originate fortuitously. What is the reason? Because previously I did not exist, but now I am. Not having been, I sprang into being.'

Brahmajāla Sutta: The All-embracing Net of Views

So generally this phenomena, in all it's small and larger appearances, not at least by such so guided practice and teachings, will increase. And (provoking), where should all the trees have gone to in this days :-)

Such as classical taught Zen-ways in modern times, put away of all thinking, is a good chance to reach the asanna states by zazen. Not being aware of anything, one might believe "coming and going, from and to emptiness"

Brahma Realm of Perceptionless Beings (Asañña-Satta Brahma-Loka), kindly given by Bhante Paññobhāsa.

--This Realm presumably is the destination of contemplatives who gain 4th jhāna without any corresponding insight; it is a kind of heavenly dead end. The beings here are virtually unconscious, and remain motionless like statues for many billions of years. After approximately 500 world cycles (the same lifespan as in Realm 21) a thought finally arises in their mind, and they immediately vanish from this strange Heaven and reappear somewhere else. Aside from the fact that they gain no benefit from this, there is the philosophical question of whether Perceptionless Brahmas are really there at all, as of course they are completely oblivious to their environment, and would experience absolutely no passage of time while in this non-state. They would seem to be inanimate objects, or at best comatose bodies. At any rate, this is an example of a high Heaven Realm that is not all that great of a place to go to—skillful meditators may as well avoid it.

Might it help in the desire to get an releasing answer: leading to disenchantment in regard of conditioned things and issues not leading to the unconditioned, by seeing the dangers in this world.

Practical, to prevent from such existences, approaching the wise, and ask

'What is skillful, venerable sir? What is unskillful? What is blameworthy? What is blameless? What should be cultivated? What should not be cultivated? What, having been done by me, will be for my long-term harm & suffering? Or what, having been done by me, will be for my long-term welfare & happiness?'

...makes one being not so much in danger of unbenefical states to gain and then just taken on this, act-ually walk for beyound.

[Note: This is a gift of Dhamma, not meant for commercial use or other lower wordily gains by ways of exchange or trade]