"Corinna, where is your new dress?"
Translation:Corinna, ubi est stola tua nova?
It's called a predicate nominative. The nominative is required with the verb 'to be' on both sides. You can think of the verb 'to be' as an equal sign: nominative + EST = nominative. Your instinct to want a direct object accusative is correct when dealing with many verbs. In this case there is only one nominative with the question. The verb 'to be' is sometimes omitted. Here's a link: http://www.carmentablog.com/2014/04/28/predicate-nominative/