。
She shut herself in the study and cried her heart out.
Literal
She [topic-は] study [in-に] shutting-herself-in with-abandon cried [explanatory-のである].
閉じこもる (to shut oneself in, to seclude oneself) creates the private space for an emotional release. 思い切り (with abandon, to one's heart's content) intensifies the crying — she didn't just cry, she cried without holding back. The sentence-final のである is the formal explanatory copula, lending the sentence a narrative or literary tone, as though explaining the significance of this moment in a story.