'Something missing' can cover a lot of things! I'm sure you have asked if she knows what it is, and she doesn't know herself.
So, several possibilities of the many:
She doesn't feel loved or able to give love
She has no goals
She feels she has not achieved anything
She has no self-esteem
She has nothing to believe in (religious or other)
If she has had a tough life she may feel tired and drained of emotion.
Not very helpful, I'm afraid, because I don't know her, but maybe you could use them as conversation starting points to help her find out what the problem is.
(((HUGS)))
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Dubbed Queen of the Illuminati by Diogenes.