。
She was never happy.
Literal
She [topic-は] never happiness was-not.
幸福 ('happiness') is the more abstract, bookish synonym of the everyday 幸せ. Both work as nouns and as な-adjectives, but 幸福 belongs to a register of essays, formal speeches, and literary narration; 幸せ is the word you'd hear at the dinner table. Negating with 決して...ではなかった ('was never') describes a sustained absence of happiness — a backstory line in a biography or novel, the kind of statement that sets up a contrasting present.