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

Here i have created sample. Refer the below code and change as per your requirement.

HTML

<div>
    <asp:GridView ID="grdview1" runat="server" AutoGenerateColumns="false" ScrollBars="Auto"
        EmptyDataText="Sorry, No DATA !" BackColor="White" BorderColor="#CC9966" BorderStyle="None"
        BorderWidth="1px" CellPadding="4" 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">
                <ItemTemplate>
                    <%# Container.DataItemIndex + 1 %>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Name">
                <ItemTemplate>
                    <asp:Label ID="lblEmpName" Text='<%# Eval("EmpName") %>' runat="server" />
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Designation">
                <ItemTemplate>
                    <asp:Label ID="lblDesignation" Text='<%# Eval("Designation") %>' runat="server" />
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Comapny">
                <ItemTemplate>
                    <asp:Label ID="lblComapny" Text='<%# Eval("Comapny") %>' runat="server" />
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Message" runat="server">
                <ItemTemplate>
                    <asp:TextBox ID="txtMessage" runat="server"></asp:TextBox>
                    <asp:Button ID="btnSendMessage" Text="Send Message" runat="server" OnClick="SendMessage" />
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
</div>

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        DataTable dtEmployee = new DataTable();
        dtEmployee.Columns.AddRange(new DataColumn[3] { new DataColumn("EmpName"), new DataColumn("Designation"), new DataColumn("Comapny") });
        dtEmployee.Rows.Add("John Hammond", "Developer", "Excelasoft");
        dtEmployee.Rows.Add("Mudassar Khan", "Owner", "Excelasoft");
        dtEmployee.Rows.Add("Suzanne Mathews", "Marketing Manager", "Excelasoft");
        dtEmployee.Rows.Add("Robert Schidner", "Team Leader", "Excelasoft");
        grdview1.DataSource = dtEmployee;
        grdview1.DataBind();
    }
}

protected void SendMessage(object sender, EventArgs e)
{
    Button btnMessage = sender as Button;
    string empName = (btnMessage.NamingContainer.FindControl("lblEmpName") as Label).Text.Trim();
    string designation = (btnMessage.NamingContainer.FindControl("lblDesignation") as Label).Text.Trim();
    string company = (btnMessage.NamingContainer.FindControl("lblComapny") as Label).Text.Trim();
    string message = (btnMessage.NamingContainer.FindControl("txtMessage") as TextBox).Text.Trim();
    string employeeDetails = "Employee Name: " + empName + "\nDesignation: " + designation + "\nCompany: " + company + "\nMessage: " + message;
    //Code to send these details as message.
}

Screenshot