。
She worked hard.
Literal
She [topic-は] with-all-one's-might worked.
A skeletal subject-adverb-verb sentence. 一生懸命 attaches to 働く without any particle — 四字熟語-derived adverbs typically modify verbs directly, though 一生懸命に is also acceptable in slightly more careful registers. The simple past 働いた reports a completed action without elaboration; the absence of any softener like ~んです gives a flat, informational tone.