<asp:DropDownList ID="DropDownList1" runat="server"
        DataSourceID="A" DataTextField="Regisno" DataValueField="Regisno">
    </asp:DropDownList>

 <asp:AccessDataSource ID="A" runat="server" DataFile="~/App_Data/Database.mdb"
            SelectCommand="SELECT [Regisno] FROM [tblsignupv] WHERE ([Email] = ?)">
            <SelectParameters>
                <asp:SessionParameter Name="Email" SessionField="User" Type="String" />
            </SelectParameters>
        </asp:AccessDataSource>

DropDownList1.SelectedValue="123";

SqlCommand comm = new SqlCommand();
comm.Connection = new SqlConnection("Data Source=(local);Initial Catalog=[db];user=[user];password=[pwd];");
String sql = @"SELECT [Regisno] FROM [tblsignupv] WHERE ([Email] = '[emailaddress]')";
comm.CommandText = sql;
comm.Connection.Open();
SqlDataReader cursor = comm.ExecuteReader();
while (cursor.Read())
{
     // add your option to the combobox here
     customerID.Items.Add(new ListItem(cursor["text"].ToString(), cursor["value"].ToString()));
}
comm.Connection.Close();