1) What is the essence?
Hard to answer, and it probably means different things to different people. But, when I get to know someone's essence, it's usually a core behavior or pattern of behavior, or need, or appreciation that guides their major life decisions as well as the smaller decisions. One friend I have is a preserver. She preserves memories, books, experiences, people. She works as a librarian. I consider myself an observer and jester.
2) How it could be missing?
It's never missing, but it could be hard to see.
3) How can you find it if it isn't there?
Either seeing it in yourself or others involves connecting to your situation as it is now, however unsatisfactory. Once you practice that that, your mind opens up and you can be intimate with people. All it takes is intimacy and perspective.