。
She suddenly fell ill on her day off.
Literal
She [topic-は] day-off [on-に] suddenly sick became.
突然 ('suddenly, abruptly') is the more dramatic and slightly more formal partner to 急に, both meaning 'suddenly.' The kanji breakdown helps: 突 ('thrust, pierce') + 然 ('the way it is') paints a sudden bursting-through, more vivid than the literal 'urgently' of 急に. In casual conversation 急に is far more common; 突然 lifts the register and is often the choice for narration, news, or emphasis on shock. 病気になる is the standard idiom for falling ill, with the implication of acquiring an illness rather than carrying a chronic one.