。
She was so shocked she couldn't speak.
Literal
She [topic-は] shock [from-excess-of-のあまり] mouth [subject-が] couldn't-speak.
口が利けない ('can't speak,' literally 'the mouth can't function') is the standard idiom for being struck speechless. 利く ('to function, work, be effective') is here in its potential 利ける, then negated and put in past as 利けなかった. Often written 口がきけない without the kanji 利, since the kanji is uncommon in this idiomatic sense. Pair with ~のあまり ('out of excessive X') and you get a vivid picture of being overwhelmed beyond words. The body part (口) takes が as the subject of the inability — a hallmark of body-part predicate constructions in Japanese, where the relevant body part is grammatically promoted to subject status.