。
She was deeply immersed in her work.
Literal
She [topic-は] work [target-に] immersion was-doing.
没頭する ('to be absorbed, immerse oneself, throw oneself into') is the slightly more elevated cousin of 夢中になる. The compound 没頭 splits into 没 ('sink, drown, immerse') + 頭 ('head') — literally 'sinking one's head into,' a vivid image of total focus. The に marks what one is immersed in. Where 夢中 leans toward emotional infatuation, 没頭 leans toward intellectual or task-focused absorption — useful for studies, work, hobbies, projects.