「？」。
"What's wrong?" the little white rabbit asked.
Literal
"Somehow did [explanatory-question-の]?" [quotative-と] small white rabbit [subject-が] asked [polite-past-ました].
どうかする, literally 'to do something,' colloquially means 'to be wrong / something is off.' どうかしたの? is the idiomatic 'what's wrong?' or 'is something the matter?' The rest shows dialogue attribution in Japanese narrative: quote + と + verb of speech, here 聞きました ('asked'). The relative clause 小さな白い modifies ウサギ (white rabbit) — note that 小さな is a 連体詞 (an adjectival noun used pre-nominally only).