。
She was wearing a silk dress.
Literal
She [topic-は] silk [genitive-の] dress [object-を] was-wearing.
シルク and ドレス are both fully naturalized English loanwords; native equivalents 絹 (kinu) and 服 (fuku) exist but feel either stuffier or less specific. Note that 着ていた is the resultant-state continuous — Japanese describes 'was wearing' (continuing state of having put on) with the same form as 'is wearing,' rather than treating the dressing as an instantaneous change. The verb choice is also significant: 着る is reserved for items that cover the torso, paired with 履く (lower body or footwear), かぶる (head), かける (glasses), and する (small accessories). Picking the wrong wearing verb is one of the giveaway tells of an early-stage learner.