If you are taken unawares by something unexpected, you can describe that feeling with the adjective surprised. Look surprised when your friends plan a party without telling you, even if you see the cars in the driveway and know what's going on.
Surprised originally meant attacked unexpectedly. It was from the Old French surprendre meaning to overtake, with the root prendere from the Latin word for seize. Surprised has synonyms in words like astonished, amazed, and flabbergasted, but the latter three convey a greater sense of shock or bewilderment. You are surprised by the party, amazed that your old friend came, and flabbergasted to learn she is actually your sister.