To fall silent or sink into silence; to stop speaking, often suddenly or for a while.
その知らせを聞いて彼は黙り込んだ。
He fell silent when he heard the news.
会議中、彼女は急に黙り込んでしまった。
During the meeting, she suddenly became silent.
子供が黙り込んだので、理由を確かめに行った。
The child went quiet, so I went to check why.