我不知道如何解释他的失踪。

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 competely ___at ___a ___loss what to write next.

当她面临这个艰难的决定时，她不知所措。

She was ___at ___a ___loss when faced with the difficult decision.