。
She vanished into the crowd.
Literal
She [topic-は] crowd [into-に] blended-in [then-て] became-invisible.
Two expressive verbs combine via the て-form. 紛れる (まぎれる) means 'to blend into, to disappear among' — more vivid and literary than 消える ('to disappear'). Then 見えなくなった breaks down as 見える ('be visible') → 見えない ('not visible') → 見えなくなる ('become not-visible'). The ~なくなる pattern marks a transition into a negative state — something that was the case ceases to be.