。
She looked around but couldn't see anything.
Literal
She [topic-は] surroundings [object-を] looked-around [but-が] nothing was-not-visible.
見回す (to look around) is a compound verb: 見る (look) + 回す (turn around) — literally 'turn one's gaze around.' The が marks the contrast between the effort (looking around) and the result (seeing nothing). 何も見えなかった uses the potential negative of 見える (to be visible) — things weren't visible, regardless of her effort to find them.