。
She was unconscious for a full day after the accident.
Literal
She [topic-は] accident [possessive-の] after, whole-one-day unconscious was.
丸 (まる) as a prefix to a number-counter expression intensifies it to 'a whole, a full': 丸一日 ('a whole day, a full day'), 丸三年 ('a full three years'). The 丸 originally means 'round, whole' and contributes the sense of completeness. 意識不明 is a fixed compound meaning 'unconscious, in a coma' — 意識 ('consciousness') + 不明 ('unclear, unknown') = 'consciousness unaccounted for.' Standard medical and journalistic vocabulary. The bare measure 丸一日 sits adverbially without a particle.