这位小友,
at a loss what to do 中 what to do是at a loss 的补语
相同的表达法也可以是：
at a loss about what to do at a loss to know what to do
at a loss as to what to do at a loss for what to do
as to for about 在这里都是关于的意思.单独的一个as,不能这么用.