nonplussed
So confused or surprised that you don’t know what to do.
Nonplussed means so surprised, confused, or puzzled that you don't know what to say or do next. When you're nonplussed, you're caught completely off guard, like when a teacher announces a pop quiz on material you’ve never studied, or when someone gives you an answer that makes absolutely no sense.
If your friend starts speaking to you in a language you've never heard before, you'd be nonplussed. If you open your lunchbox to find someone has replaced your sandwich with a rubber chicken, you'd probably stand there nonplussed for a moment.
Being nonplussed is temporary: it's that frozen moment of “wait, what?” before you figure out how to respond. A student might be nonplussed by a challenging riddle, standing silently while their brain tries to catch up. The word captures that specific feeling of being stumped, baffled, and at a loss all at once.
Note: Some people mistakenly use nonplussed to mean calm or unbothered, which is actually the opposite of its traditional meaning. If you see it used this way, check the context carefully.