To be speechless; to be at a loss for words because of surprise, shock, sadness, etc.
彼の突然の告白に私は言葉を失った。
I was speechless at his sudden confession.
そのニュースを聞いて家族は言葉を失ってしまった。
Upon hearing the news, the family was at a loss for words.
あまりの美しさに思わず言葉を失った。
I was left speechless by the sheer beauty.