Refresh the parent page from popup window,
Then Bind the dropdownlist items on !is post back.
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
SqlConnection con = new SqlConnection("Data Source=;Database=;User ID=;Password=!;Asynchronous Processing=true");
con.Open();
SqlCommand cmd = new SqlCommand("select firstname from contacts", con);
SqlDataAdapter dtAdapter = new SqlDataAdapter(cmd);
DataTable dt= new DataTable();
dtAdapter.Fill(dt);
ddl1.DataSource = dt;
ddl1.DataTextField = "firstName";
ddl1.DataValueField = "firstName";
ddl1.DataBind();
ddl1.Items.Insert(0, new ListItem("select", "0"));
}
}