。
I was caught completely off guard.
Literal
Surprise-attack [object-を] took.
不意打ちを食らう is a vivid set expression — 不意打ち ('surprise attack') combined with 食らう ('to take a hit, to suffer'), a blunt, earthy synonym of 受ける. The subject is entirely omitted, which is natural when context makes it clear. The conciseness gives the sentence a punchy, spoken feel.