I'm speechless
I'm so shocked, bewildered, or overwhelmed that I can't articulate my thoughts coherently or speak at all right now. A: "Will you marry me?" B: "Oh my God, oh my God, I'm speechless! Wait, yes, yes! Of course!" A: "Can you believe he said something that nasty to me?" B: "No. I'm speechless."
I'm speechless.
Fig. I am so surprised that I cannot think of anything to say. Mary: Fred and I eloped last week. Sally: I'm speechless. Tom: The mayor just died! Jane: What? I'm speechless!