The phrase "struck speechless" refers to a situation where someone is so surprised or shocked that they are unable to speak. There are several synonyms that can be used to convey the same meaning, such as dumbfounded, flabbergasted, stupefied, astounded, and stunned. Each of these words brings a slightly different nuance to the concept of being speechless. For example, "dumbfounded" implies a complete loss of words, while "stunned" suggests a physical reaction to the shock. "Flabbergasted" is a more informal term that conveys a sense of being overwhelmed, and "stupefied" suggests a temporary mental paralysis. All of these words can help to convey the idea of being completely taken aback and rendered speechless.