。
She had her hair cut short.
Literal
She [topic-は] hair-strand [object-を] short cut [had-someone-do-てもらいました].
髪の毛 ('hair') is a more colloquial, slightly redundant way to say 髪 — literally 'the hair-strand of (one's) head.' Polite ~ました ending shifts this into formal speech. The ~てもらう construction is the standard way to talk about getting a haircut.