。
She died of suffocation.
Literal
She [topic-は] oxygen-deficiency [from-で] died.
酸欠 ('oxygen deficiency, asphyxiation') is a Sino-Japanese compound built from 酸 ('oxygen, acid') + 欠 ('lack, deficiency') — short for 酸素欠乏 ('oxygen lack'). Common in technical and emergency contexts, especially industrial safety reports about confined-space accidents. The で marks the cause of death, the standard pattern for fatal causes: 病気で死ぬ ('die of illness'), 事故で亡くなる ('die in an accident'). 死ぬ is the bare, direct verb for dying — 亡くなる is the polite/euphemistic alternative used for people one wishes to speak respectfully about.