Dear Hari,
Non existence is a situation where an object or world or even a person is not existent all the time, eternally. The object,
the world, and a person has a birth, life and death. These are not eternal. Whereas Consciousness is eternal and everpresent,
born, and non dying.
In sleep your thoughts disappear, mind disappears, the world disappears, the time and space concept
also disappear. Still you say that you have slept well, next morning. This is state of being 'unconscious', though the
Self within you is ever there, without sleeping, ever vigilant. Similarly during fainting and other temporary states where
mind is in suspended animation, you are still there, because Consciousness is within you.
Arunachala Siva.