▲at a loss是介词短语，也是英语的习惯用语，意为“感到困惑；不知所措”，之后经常跟wh-word + to do形式。由于表达“疑问”的概念，所以后面的从句不用that引导，而是用带有疑问的wh-word引导。at a loss直接跟wh-word从句的情况较少（如：She was at a loss what she should do. 她不知该怎么办好。）而是wh-word + to do居多，例如：

She is timid and completely at a loss what to do. 她羞臊得不知所措。

The question was so unexpected that , for a moment , she was at a loss how to act up properly. 问题来得这么突然, 她一时不知道如何作出恰当的反应。

Mary is at a loss when to start. 玛丽不知道何时开始。

He was at a loss how to cut the knot. 他不知如何才能大刀阔斧地解决这一难题。

Facing risks, human beings are not at a loss what to do. 面对风险, 人类并非处于亏损该怎么办。

She was at a loss how to cope with the situation. 她不知如何去应付这种局面。

The enemy was entirely at a loss as to how to cope with us. 敌人对我们很没有办法.。

He is always at a loss what to say in front of strangers.他在生人面前总是不知要说什么。

at a loss之后直接跟动词不定式：

I am always at a loss to know how much to believe of my own stories.我老是在困惑，对自己写的那些故事究应相信到什么程度。

One is at a loss to suggest a possible improvement.人们不知该如何提出可能的改进方法。

I'm at a loss to know what to do.我不知道干什么好。

She was at a loss to give words to indignation. 她简直不知道该如何用语方来表达她的义愤。

at a loss之后可以看做省略了as to（对于，关于），以下是含有as to的句子：

I am at a loss as to what happens now. 我对现在的事情,真有点儿不摸头。

They were at a loss as to how to meet the situation. 他们手足无措,不知该如何应付这一局面。

She was at a loss as to how to explain it. 她全然不知道该如何说明这件事。

James was at a loss as to what to do and felt lost. 詹姆斯无所适从,惘然若失。

Many people are at a loss as to what to do in case of a real fire.一旦真的发生火灾，许多人就不知所措，不知道该干什么好了。

▲关于 I was completely at a loss what to do 中，what to do做何种成分的问题。我做以下解释，都可以接受：

1. 把what to do看作是be at a loss的宾语，把be at a loss理解为及物动词wonder，用wonder取而代之。

2. 在at a loss之后视为省略了介词as to，上面已有例句。

3. 把what to do看作是loss的同位语，类似于：

I have no idea what you mean. 我一点儿也不明白你的意思。

I have no idea who decided the composition of the committee? 我不知道委员会的成员是由谁选定的。