。
She was speechless with shock.
Literal
She [topic-は] shock [due-to-で] mouth [also-も] couldn't-speak.
口もきけない is the speechless-with-shock idiom. で here is the causal で — '(she couldn't speak) because of the shock.' The choice of も over the more neutral が intensifies the helplessness: it's not just that her mouth wasn't working — even her mouth (of all things) wasn't working. The kana spelling きけなかった is the typical form; with kanji it would be 利けなかった, where 利く means 'to function, work properly.' A vivid frame for being so overwhelmed that words simply don't come.