。
Her hat got blown off yesterday.
Literal
She [topic-は] yesterday hat [object-を] was-blown-off (to her detriment).
This is the classic adversative passive (迷惑の受身) — even though the wind, not the speaker's hat, is the affected object grammatically, Japanese frames the woman herself as the passive subject who suffered the event. The hat is marked with を (its possessor's belonging), but she is the topic carrying the impact. English handles this kind of inconvenience-passive with longer phrasing ('she had her hat blown off,' 'her hat got blown off on her'). 吹き飛ばす is itself a compound verb (吹く 'blow' + 飛ばす 'send flying'), packing a vivid two-stage action into a single word.