If you are nonplussed, something has surprised you so much that you stop. You do not know what to say or what to do next. The old idea in the word is “no more”: you cannot go any further; you are stuck. Books, news, and teachers still use it this way. Though actually, in everyday American English many people now use it to mean the opposite — not bothered at all, as if nothing special happened. That newer use is probably from a misreading of non- as a simple “not,” so people guessed it meant “not affected.” Both uses are out there, so if someone says “I was nonplussed,” you may not know if they were lost for words or completely calm. The older meaning sounds a little formal and bookish; the newer one is more casual US talk.

不知所措、愣住。美式口語有人用來表示「不為所動」，意思正好相反。