。
She was speechless at the news.
Literal
She [topic-は] that news [at-に] was-dumbstruck.
唖然とする ('to be dumbstruck, stand agape, be at a loss for words') uses the 四字熟語-flavored compound 唖然 — 唖 means 'mute' and 然 is a state-suffix giving 'in a mute state.' The expression conjures someone literally unable to speak from shock or astonishment. と sets the manner adverbially before する, the same と that pairs with mimetic adverbs (じっと, ぼんやり). に marks the trigger. A compact, vivid choice for written narration of stunned silence.