。
She refused to talk about what had happened.
Literal
She [topic-は] what [subject-が] happened [embedded-のか] talking [object-を] refused.
Two の's stack here, doing different work. The first turns 何が起こった into an embedded wh-question with のか ('what had happened'), serving as the topic of the talking. The second nominalizes 話す ('talking') with の to make it the object of 拒んだ. Stripping away the layers: she + refused + the act-of-talking + about-what-happened. Japanese piles these clausal nominalizations cleanly with no help from punctuation or 'that.'