So surprised and confused that one is unsure how to react.
In standard use nonplussed means ‘surprised and confused’, as in she was nonplussed at his eagerness to help out. In North American English a new use has developed in recent years, meaning ‘unperturbed’—more or less the opposite of its traditional meaning—as in he was clearly trying to appear nonplussed. This new use probably arose on the assumption that non- was the normal negative prefix and must therefore have a negative meaning. It is not considered part of standard English