。
She was never happy.
Literal
She [topic-は] never happy was-not.
幸せ is the everyday word for happiness — the one you'd hear at the dinner table or read in a song lyric, distinct in register from the bookish synonym 幸福 (which lives in essays and reflective narration). 幸せ functions both as a な-adjective (幸せな人 'a happy person') and as a noun (幸せを願う 'wish for happiness'). With 決して...ではなかった layering on the strong negation, the line lands as something a friend might say in casual sympathy: she just wasn't happy, plain and simple.