ExecuteNonQuery 要求已打开且可用的连接。连接的当前状态为已关闭。

protected void lbtnDelete_Command(object sender, CommandEventArgs e)
    {
        if(Session["admin"]!=null)
        {
      
            string userID = ();
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = DB.getConnection();
             = "delete from guest where ID='"+userID+"'";
            if (cmd.ExecuteNonQuery() > 0)
            {
                Response.Write("<script>alert('删除成功！');window.location=window.location;</script>");
            }
            else
            {
                Response.Write("<script>alert('删除失败！');window.location=window.location;</script>");
            }
        }


运行时出现“ExecuteNonQuery 要求已打开且可用的连接。连接的当前状态为已关闭。”这样的错误，是怎么回事呢 代码要怎么写啊，请各位帮我看看，谢谢！