The phrase "not know what hit you" implies being taken by surprise or being completely shocked by something unexpected. There are many different synonyms for this phrase that one could use such as "caught off guard", "blindsided", "knocked for a loop", "stunned", "dumbfounded", "flabbergasted", "surprised", "astonished", "amazed", "perplexed", "bewildered" and "taken aback". Each of these phrases conveys the sense of being utterly surprised by something unexpected, but each also carries its own nuance and connotation, so it's important to choose the right one for the context in which it's being used.