。
She was so scared she couldn't speak.
Literal
She [topic-は] scared [so-て], mouth [subject-が] was-not-able-to-function.
口がきけない is a set expression for being unable to speak — literally 'the mouth doesn't work.' The verb 利く here means 'to function, to be effective' (the same 利く you get in 気が利く 'to be considerate' or 鼻が利く 'to have a good nose'); paired with 口, it's the metaphor of the mouth as a working organ that's stopped working. Note that the patient (口) is marked が, not を, because the predicate is the potential form: 口を利く is the active idiom ('to speak'), and 口が利けない is its potential negative — Japanese reroutes the marker as part of the potential's grammar. The phrase often describes not just shock but being stunned, intimidated, or rendered speechless by overwhelming emotion.