Speechlessly is the adverb of speechless: doing something while too shocked, angry, or moved to say anything. It describes HOW an action is done, so it needs an action verb next to it — you stare speechlessly, nod speechlessly, sit speechlessly, watch speechlessly, hand something over speechlessly. Notice the shape of that list: the actions are all silent ones. That is not an accident. Since the word means no words come out, it attaches to the body doing the talking instead — eyes, head, hands. This is a distinctly written word. It belongs to novels and reports of dramatic scenes; in real speech people simply say 'I was speechless' and then say what they did. If you are a student writing a story, it is a good word to have. If you are chatting with a friend, you will almost never need it. Do not confuse the shape with the far more common pattern 'be speechless' — the adjective, not the adverb, is what English uses most of the time.

啞口無言地、說不出話地（描寫動作，如「他啞口無言地看著我」）。這是書面語，寫小說、寫記敘文用；平常講話直接說 I was speechless 就好。