A break in contact; having no communication with someone — not hearing from someone at all.
彼とは卒業以来ずっと音信不通だ。
I haven't heard from him since graduation.
台風で電話が通じず、家族と数日間音信不通になった。
The typhoon cut phone service, and we were out of contact with our family for several days.
取引先が突然音信不通になり、状況がわからない。
The client suddenly stopped communicating, and we don't know the situation.