。
She was at a loss for words.
Literal
She [topic-は] words [at-に] became-clogged.
言葉に詰まる ('to get stuck on words') is the standard idiom for the moment when words stop coming — surprised silence, embarrassed pause, or the speechlessness of strong emotion. 詰まる literally means 'to be stuffed, clogged, jammed' (パイプが詰まる, 'a pipe is clogged'), and the metaphor maps speech onto a flow that has hit an obstruction. The に marks the realm where the clogging happens: stuck *at* words. The past tense indicates a single moment when this happened, not an ongoing trait.