。
She was surprised at the sight.
Literal
She [topic-は] that scene [target-に] was-surprised.
~に驚く ('be surprised at') is the canonical frame for surprise, with に marking the trigger of the emotional reaction. The plain past 驚いた describes the discrete moment of surprise — distinct from the progressive 驚いていた ('was [still] surprised'), which would frame the surprise as a sustained state. Note the consistent pattern across emotion verbs in Japanese: に marks the source of the feeling for 驚く, 感動する, 感心する, ぞっとする, and others — Japanese treats the trigger of an emotion as a target rather than an agent.