Find TextBox value inside GridView on Button click using C# in ASP.Net

<asp:GridView ID="grdview1" runat="server" AutoGenerateColumns="true" ScrollBars="Auto"
 EmptyDataText="Sorry, No DATA !" BackColor="White" BorderColor="#CC9966" OnRowCommand="grdview1_RowCommand"
 BorderStyle="None" BorderWidth="1px" CellPadding="4" OnRowEditing="grdview1_RowEditing"
 CssClass="mGrid">
 <FooterStyle BackColor="#FFFFCC" ForeColor="#330099" />
 <PagerStyle BackColor="#FFFFCC" ForeColor="#330099"  HorizontalAlign="Center" />
 <SelectedRowStyle BackColor="#FFCC66" Font-Bold="True" ForeColor="#663399" />
 <Columns>
    <asp:TemplateField HeaderText="SerialNo" runat="server">
       <ItemTemplate>
          <%# Container.DataItemIndex + 1 %>
       </ItemTemplate>
    </asp:TemplateField>
    <asp:TemplateField HeaderText="Message" runat="server">
        <ItemTemplate>
           <asp:TextBox ID="txtmsg" runat="server"></asp:TextBox>
        </ItemTemplate>
    </asp:TemplateField>
 </Columns>
</asp:GridView>

Hi i want to add one textbox in already binded gridview for sending message, i am binding data of family survey information by using checkbox,what are the thjings i want i checking checkbox, which i have checked that are binding in below grid,in that grid i want textbox and one button at the end gridview rightside,so if u have any idea please help me

Thanks & regards

Umesh Naik