was at a loss as to what to doUsed after a question word such as どうして, 何を, or どうしたら to express being unsure what to do, say, or choose. いいか途方に暮れた is the more conversational form.
JA: 突然質問されて、何と答えてよいか途方に暮れた。
EN: I was at a loss for what to say when I was suddenly asked a question.