。
She acted as if she didn't care in the slightest about what had happened.
Literal
She [topic-は] what [subject-が] happened [embedded-か] entirely is-not-minding [as if-かのように] behaved.
まるで~かのように is the canonical 'just as if' construction, with まるで ('exactly, completely') intensifying the counterfactual simile. 気にする ('to mind / to be bothered by') paired with the negative ~ていない form expresses an ongoing absence of concern. 何が起きたか folds an embedded question inside the larger clause as the thing she is not minding. Stack it all up and the sentence describes elaborate, performed obliviousness.