Bhagavad Gita, Chapter 2, Verse 28

The source of things, O descendant of Bharata, is unperceived; their middle state is perceived; and their end again is unperceived. What is there then to grieve about?

Every being is unseen before birth, seen during life, unseen again after death. None of that is worth grieving.

Nothing about a person actually vanishes at death, only the visible part of the cycle changes. What can be seen was never the whole story to begin with, so its disappearance isn't really a loss.