。
Why did she suddenly stop talking?
Literal
She [topic-は] why story [object-を] suddenly stopped [explanatory-のか].
なぜ is the formal/written interrogative for 'why' — slightly more written or interrogative-pointed than the conversational どうして or なんで. The sentence ends with のか, which combines the explanatory の + question か, giving 'why is it that X?' This のか framing softens the bare question into a more reflective, almost rhetorical one — common when the speaker is genuinely puzzled and seeking explanation rather than a yes/no answer. 急に ('suddenly') is the standard adverbial of 急 (a na-adjective meaning 'urgent, sudden').