我从密码修改界面返回学生界面为什么datatable里面的数据没了？？？求大神解答

private void 学生界面_Load(object sender, EventArgs e)
{

        string connstr = "Server=.;database=Student1;uid=sa;pwd=xld158";
        SqlConnection connection = new SqlConnection(connstr);//数据库连接实例
        connection.Open();
        SqlDataAdapter da = new SqlDataAdapter();
        SqlCommand b = new SqlCommand("select name,number,sex,phone,sic,ability from StuMess where userid='" + d + "'", connection);
        da.SelectCommand = b;
        DataTable dt = new DataTable();
        da.Fill(dt);
        dataGridView1.DataSource = dt;
        label8.Text = dt.Rows[f]["name"].ToString();
        label9.Text = dt.Rows[f]["number"].ToString();
        label10.Text = dt.Rows[f]["sex"].ToString();
        label11.Text = dt.Rows[f]["phone"].ToString();
        label12.Text = dt.Rows[f]["sic"].ToString();
        label13.Text = dt.Rows[f]["ability"].ToString();
        connection.Close();
    }
                              public 密码修改界面()
    {
        InitializeComponent();
    }

    private void button2_Click(object sender, EventArgs e)
    {

        学生界面 form2 = new 学生界面();

        form2.Show();
        this.Close();
    }

每次请求页面，都会执行对应页面的page_load，作为page_load的局部变量的dt，自然每次都需要重新填充一次，之前的无效