。
She lost her mind after her son's death.
Literal
She [topic-は] son [genitive-の] after-death mind [subject-が] went-mad.
息子の死後 uses the suffix ~後 (ご, after a noun) to mark 'after [event].' 死 ('death') as a bare noun is formal/written; the more colloquial alternative is 亡くなった後 ('after [he] died'). 気が狂う ('go mad / lose one's mind') here applies in the literal sense — actual loss of mental stability rather than the metaphorical loss-of-composure reading the idiom often carries. The combination of grief and 気が狂う portrays a tragic descent into madness, a recurring theme in Japanese fiction and historical accounts.