。
The hat got blown away.
Literal
Hat [subject-が] blow-away-[passive-された]
吹き飛ばす (blow-fly = blow away) in passive: 吹き飛ばされた. Note が rather than は — the hat is presented as new information (perhaps a sudden event), not a topic already under discussion. No agent expressed; Japanese passives often leave the doer unmentioned when it's obvious (here, the wind).