我用你的代码 一直不能成功 新密码怎么传入啊

ConnectionString 属性尚未初始化。在第二个 con.Open();
                                          cmd.ExecuteNonQuery();
                                          con.Close();  错误
源代码如下

  SqlConnection con =new SqlConnection();
            SqlCommand cmd=new SqlCommand();
            cmd.Connection=con;
            "select UserName,UserPwd from Users where UserName='"+this.txtUName.Text+"'";
            string uname="";
            string pass="";
            try
            {
                   
               con.Open();
               SqlDataReader reader=cmd.ExecuteReader();
               if(reader.Read())
                   {
                    uname=reader[0].ToString();
                    pass=reader[1].ToString();
                   }
               reader.Close();
               con.Close();
            }
            catch{}
            if (uname.Trim() == this.txtUName.Text)
            {
             MessageBox.Show("对不起，您输入的用户不存在！");
             return;
             }
            if (pass == this.txtPass.Text)
            {
                MessageBox.Show("对不起，");
                return;
            }
             if(pass.Trim()==this.txtPass.Text)
              {
                 MessageBox.Show("对不起，用户"+uname+"的密码不正确！");
                  return;
             }
             if(this.txtPass1.Text!=this.txtRePass.Text)
             {
                  MessageBox.Show("对不起，您输入的两次密码不一致");
                  return;
             }
             string sql="update Users set UserPwd='"+this.txtPass.Text+"' where UserName='"+uname+"'";
             try
             {
               con.Open();
               cmd.ExecuteNonQuery();
               con.Close();
             }
             catch(SqlException er)
            {
               if(con.State==ConnectionState.Open)
                  con.Close();
                  MessageBox.Show("无法修改密码！\n"+er.Message);
                  return;
             }
             MessageBox.Show("已成功修改密码！");