It's fine that you mentioned it here.
It's a terribly sad story, though. Most kids think they're the most interesting person in the world and that everything that comes out of their mouths is brilliant.
From what I hear one of the things that might has set this girl's problems off is that a new nanny was told that she was old enough to take care of herself and that most of the attention would need to go to little sibling. In the past, the nannies were her companions. I can see lack of attention contributing to this kind of silence.
The fact that this was so sudden is what makes it so strange.