。
No one knows what became of her.
Literal
She [topic-は] how became [explanatory question-のか] no-one knows.
An embedded question with のか functions as the object of 知らない: '[the question of] what became of her, [no one] knows.' The のか version (versus a bare か) carries the explanatory の, treating the question as something whose answer is being sought — slightly more reflective than 誰も知らない alone. Note the universal-negation pattern 誰も~ない: an interrogative + も plus a negative predicate yields 'no one / none / nothing,' the standard frame for blanket negation.