In order to come to a reasonably clear conclusion here, we would first have to define what consciousness is. A neuroscientist would probably say that it is neurochemical processes that influence neuronal activity in the brain and lead to consciousness. This assumption is supported by numerous experiments that investigate the connection between the brain and consciousness.
Most attempts to explain consciousness are probably of a spiritual nature. However, there are also very wild theories, for example that the brain is an interface to a collective intelligence or a kind of "cloud consciousness".
In the end, however, it probably doesn't matter which conclusion you come to, because all attempts to explain it fail because of one question: How can matter lead to a thinking self? I don't think this question can ever be fully clarified; consciousness will probably remain a metaphysical mystery.
The mere fact that we have no idea what consciousness really is and how it works means that we can only make wild assumptions about the topic of "consciousness on/off", all of which of course cannot be empirically proven.
What can be considered relatively certain, however, is that things that affect consciousness, such as decision-making or reactions, can be measured in the brain. In addition, neurochemical processes, such as the interaction between neurotransmitters and receptors, have a
significant influence on consciousness, as it influences mood, sleep, what we do and many other things.
From this we can conclude that consciousness seems to be chained to our body and dependent on it.
I would like to believe that consciousness is a separate entity from the body, that's a nice thought. Unfortunately, we simply have to face the fact that this idea is no longer tenable in the light of our current scientific knowledge. We have to assume that the composition of the brain and neuronal activity lead to consciousness and thus to our ego. This would mean that an exact copy of a brain, down to the atoms, would result in the same consciousness, the same ego. Conversely, this means that our exact brain is necessary to recreate our consciousness. Unfortunately, this has consequences for many romantic ideas. Reincarnation, for example, is effectively off the table.
There are now several problems with the question of whether a consciousness could exist again and again.
Firstly, of course, consciousness itself can no longer make any changes as soon as it ceases to exist, so it would be dependent on external events. Since we can now assume that a consciousness would need the exact same brain, there is not much choice. The only plausible possibility, at least in theory, would be the cryonics already mentioned by other users, which, it has to be said, is pure science fiction dreaming. During freezing, the structure of the brain is completely destroyed by the ice crystals, rendering the brain unusable.
Of course, we don't know what other types of preservation will be discovered, and it may be that one day we will be able to place brains in liquids, for example, in order to protect the brain structure during preservation. At the moment, however, we are miles away from something like this, and whether it will ever be possible is questionable. And even if it were, the brain would have to be preserved as early as possible. As soon as it begins to decompose, more and more information is lost until it is finally completely gone. So unless you die at a time when this is possible
and right on the spot so that the brain can be preserved quickly, it is not possible, even in theory, to be resuscitated in this way.
Another important aspect of this topic is that, in theory, you have to think in terms of instances of consciousness. This has also already been mentioned here, but I would like to expand on it a little. This can be explained quite simply using teleportation: If I teleport a person from A to B, their body is disintegrated at A and reassembled from other atoms at B. This means that the teleported person now has a de facto new body, a new brain. If the composition is absolutely identical, at B he will not notice this. However, the person who was dissolved in A is dead. The person lives on, but in a new instance. This means that even if your brain was copied and you technically lived on - you, i.e. your current instance, would definitely be dead and would never come back.
TL;DR
when you die you cease to exist, does that mean that you can switch between the state of non-existence and existence and vice versa?
No. From a natural point of view, this is absolutely impossible, so it depends on the appropriate technology. Even if that may or may not be possible one day, we would need that technology before you die, so you don't need to worry.
What ensures that after death the state of non-existence will not be interrupted again?
As soon as you die, you are gone. If your brain takes physical damage or starts to decompose, any chance of revival is lost for all eternity and your consciousness will never exist again in the history of the universe.
If you die and are not revived in time, you will never think a thought again, it will all feel like the billions and billions of years before you were born: nothing.
Hope this helped.