。
She was happy for a while.
Literal
She [topic-は] for-a-while [interval-の間] happy was.
幸福 (こうふく) is the elevated synonym of 幸せ ('happy'); both translate to 'happy' but 幸福 has a Sino-Japanese, slightly philosophical or written feel — appropriate for narration or reflection on a life period — while 幸せ is more everyday and emotionally direct. The past tense 幸福だった ('was happy') paired with しばらくの間 ('for a while') creates a melancholy frame: she WAS happy, but the past tense plus the bounded duration implies the happiness ended. Often the kind of phrasing you'd find in a memoir or novel rather than casual conversation — there's an unspoken 'and then...' lingering at the edge.