不知所措
Stand at gaze.
To be at loose ends.
To be up a tree.

eg：He was at a loss as to what to do next.

中英俚语俗语来源：三国志·吴志·诸葛恪传：“哀喜交并，不知所措。”

同义词：六神无主 To be all at sea.
反义词：胸有成竹 Have an ace up ones sleeve.