Do one thing add event of dropdownlist
<asp:dropdownlist id="ddl1" runat="server" onload="dropdown1_load">
in codebehind
protected void dropdown1_load(object sender,EventArgs e)
{
SqlDataAdapter da = new SqlDataAdapter("select sno from student", cn);
DataTable dt = new DataTable();
da.Fill(dt);
DropDownList ddl = (DropDownList)sender;
ddl.DataSource = dt;
ddl.DataTextField = "sno";
ddl.DataValueField = "sno";
ddl.DataBind();
}