If you describe something as unexplained, you mean that the reason for it or cause of it is unclear or is not known.
双语例句
例： An unexplained death is difficult to come to terms with.
原因不明的死亡是难以接受的。
例： The city's water supply has been cut for unexplained reasons.
城市的供水被莫名其妙地切断了。