so i found this answer on yahoo that was relating to this question, and this was a pretty good answer. am not the source. I used to try to imagine not existing all the time when I was a child. But I also realized the first time I tried that you simply cannot imagine not existing. The act of imagening itself is proof enough of your existance. It's like trying to convince yourself that you are in the 1700's when you can hear automobiles roaring around you and you have an ipod in your hand- it simply defies logic.
SO no, you can't imagine inexistance, that is, you cannot attempt to feel how you would feel if you didn't exist, because if you didn't exist, you wouldn't be able to feel. Are you following this?
When I was younger and attempted this, I would try not to think (impossible, because, try as you mind, a thought will pop into your head: "hey i did it, i'm not thinking!"- "hey, wait a minute, did I just think? darn.") I would then try to imagine darkness, the lack of sight, the lack of feeling, hearing, taste, emotion. I would try to not be, not breath, not exist. I couldn't manage it of course, but for a couple nanoseconds it felt as though my heart had stopped. Afterwards I would feel really unsettled, cold, empty; it was an awfull feeling, accompaning the awfull thought that I would not always exist. My only comfort was that I did exist, and that if I ever stopped existing, I would not suffer because only creatures that exist have the capacity to suffer. It's one of the risks of life. :)
To answer your question, "What is it like to not exist?" :
If you don't exist, you can't feel. You have no body, no sould, no mind, no nervous system, no nerves to send electrical signals to your brains telling you whether you are cold or hot, sad or happy, or if you are holding someones hand, touching a keyboard, or floating in space. You won't be able to breath, or speak, or even articulate the thoughts that you can't formulate. You won't even be aware that you don't exist. You will have no insecurities, no desire, no pain, no dreams, no memories. You will have no self-awareness; you won't even be a "you". You will be nothing, less than a thought, less than an idea, a breeze, or a palimphest of what you once were, because all these things, however spiritual and noncorporeal, at least exist in some plane of reality. you, however, would not exist at all.
Not existing is different than being nothing, different than darkness, different than nothing or emptyness, for even emptyness is something, right? Therefore, being creatures of existance, under the illusion of space and time, we cannot comprehend inexistance. We must attach existance or being into everything we think of, or we would not be able to think about it. Our minds can only work that way, because we know nothing else; all we know is existance. If we think about darkness, we refer to it as darkness, giving it a name. If it has a name, then it exists, even if just in concept. If we think about a bunny, we have named it bunny, and accepted it's existance. If we think about God, we have accepted, if not the existance of a god, at least the existance of the belief in a god, the IDEA exists. If we think about nothing, we make nothing something, by just thinking about it, because thinking about something acknowledges it's existance, or we would not be able to think about it. Therefore, none existance exists in our plane of existance, but only as an idea. We cannot really comprehend nonexistance, we can only come close to understanding the IDEA of nonexistance, which is in itself something that exists, and is not nonexistance itself, only the idea of it. (the bunny i nyour head is not the bunny itself but the idea of the bunny, etc.)
Be carefull what you are asking, non-existance is such an incomprehensible topic, that the human mind cannot grasp it or wrap it's head around it, much less than a blind man can understand sight, if he lived in a world where all were blind.
For now, enjoy your existance, the lows and the highs, for without darkness, we would not enjoy light, and without death, we would not enjoy life. - Lo