Considering that passive voice is all about focusing on the object that is receiving the action and turning it into the subject of the sentence, we would need to identify the parts of the sentence at hand:
Subject: [a strong] wind
Verb: blow off
Object: [her] hat
Then, knowing that active voice is in a certain order (subject + verb + object), now we need to invert that order (object [now subject] + verb [now past participle] + subject [now object ]
Her hat was blown off by a strong wind.
