Members of the family tree disappears

I too all of the sudden lost half of my family tree after one person disappeard, but my first two sims are still there?! But their one of their two daughters and her side of the branch is missing? Why? If I go to the great grandchilds profile of the missing sim his grandparents (the missing sim and her husband) are grey with no information? I too have actually worked just for this family tree and I'm annoyed that there are people lost from there...