Usage patterns
平静を失うto lose one's composure
He lost his composure upon hearing the news.
平静を取り戻すto regain one's calm/composure
I took a deep breath and regained my composure.
平静な態度a calm demeanor/attitude
She maintained a calm demeanor even after the accident.
Expressions
平静を保つkeep calm; maintain composure; keep one's temper