How to find a Child taken by Social Worker

I was devastated when one of my sim children simply disappeared today. I was browsing through "Manage Households" and noticed she was gone. I went in to play another family and suddenly both children in that family got "very sad" moods because a child had been "whisked away." This little girl was never neglected, and I'm sad that she's gone. I'm just creating a storyline around it, albeit not a very happy one.

I'm just shocked and bewildered as to how and why this suddenly happened. :(