To fetch value in dropdownlist

I have to use 2 tables in dropdownlist 1) userdetail: d_id 2) department : d_id, d_name.
In gridview as i clicked on edit link it will open new form and fetch data from the userdetail table..here in form i used dropdownlist for dep. name...but i can't get the value in it.. how do i do so..??
I have coded like this..
  protected void Page_Load(object sender, EventArgs e)
    {
       

            if (Request.QueryString.Count != 0)
            {
                if (Request.QueryString.Get(0) == "modify")
                {
                 ...