。
She suddenly burst into tears.
Literal
She [topic-は] suddenly began-to-cry.
急に + 泣き出す is one of the most common applications of the inceptive ~出す compound — it captures the involuntary breaking-out of tears that English renders with 'burst into.' The pattern is highly productive across verbs of emotional reaction: any verb of involuntary feeling can host ~出す to mean 'suddenly break into.' Compared to the smoother 泣き始める ('start crying'), 泣き出す paints the moment as a sudden release, with no controlled build-up.