我不知道该怎么解释他的失踪。

I'm ___at ___a ___loss to explain his disappearance.

现在我完全不知道接下来该写什么。

And now I am completely ___at ___a ___loss what to write next.

面对这个复杂的问题时，她不知所措。

She was ___at ___a ___loss when faced with the complex problem.