I really don't know what to say,

I really don't know what to say,
I really don't know what to say,

I really don't know what to say,
我彻底无语了（仅限于好友开玩笑啊）
正式的话,看情况,可以是 我不知道该说什么
也可以是,我不知道该如何表达

我真的不知道该说什么了

我真不知如何说才好