。
She lost her son in a car accident.
Literal
She [topic-は] car-accident [in-で] son [object-を] lost.
失う ('to lose') is used both for losing objects and for losing people through death — exactly parallel to English 'lose' in this respect. The で marks the circumstance of loss. 息子 (むすこ, 'son') belongs to a small set of basic kinship terms; in your own family you say うちの息子 ('my son'), but referring to someone else's son politely you'd say 息子さん. Note how the semantic burden of this sentence sits entirely on 失った — the literal translation 'lost a son' would feel cold in English, but Japanese 失う carries the same emotional weight as English 'lost,' and in fact this exact construction is the standard way to phrase bereavement.