Hi counterkin,

Check this example. Now please take its reference and correct your code.

Database

For this example I have used of Northwind database that you can download using the link given below.

Download Northwind Database

HTML

<div>
    <asp:GridView ID="gvCustomers" runat="server" CssClass="table table-sorting table-hover table-bordered datatable" DataKeyNames="CustomerID"
        AutoGenerateColumns="false" GridLines="None" OnRowDataBound="OnRowDataBound">
        <Columns>
            <asp:TemplateField HeaderText="Admission No">
                <ItemTemplate>
                    <asp:GridView ID="gvOrders" runat="server" CssClass="table table-sorting table-hover table-bordered datatable" DataKeyNames="OrderId"
                        AutoGenerateColumns="false" GridLines="None">
                        <Columns>
                            <asp:TemplateField HeaderText="Phone No">
                                <ItemTemplate>
                                    <asp:Label ID="lblOrderId" runat="server" Text='<%# Eval("OrderId") %>'></asp:Label>
                                </ItemTemplate>
                            </asp:TemplateField>
                            <asp:TemplateField HeaderText="Pay">
                                <ItemTemplate>
                                    <asp:LinkButton ID="lnkBtnEdit" runat="server" Text="Pay Now" CssClass="btn btn-info" OnClick="Display"></asp:LinkButton>
                                </ItemTemplate>
                            </asp:TemplateField>
                        </Columns>
                    </asp:GridView>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Id">
                <ItemTemplate>
                    <asp:Label ID="lblId" runat="server" Text='<%# Eval("CustomerID") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Name">
                <ItemTemplate>
                    <asp:Label ID="lblName" runat="server" Text='<%# Eval("ContactName") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="City">
                <ItemTemplate>
                    <asp:Label ID="lblCity" runat="server" Text='<%# Eval("City") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Country">
                <ItemTemplate>
                    <asp:Label ID="lblCountry" runat="server" Text='<%# Eval("Country") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
</div>
<div>
    <div class="modal fade" id="myModal" role="dialog">
        <div class="modal-dialog">
            <!-- Modal content-->
            <div class="modal-content">
                <div class="modal-header">
                    <button type="button" class="close" data-dismiss="modal">&times;</button>
                    <h4 class="modal-title">Details</h4>
                </div>
                <div class="modal-body">
                    <div class="form-group">
                        <asp:Label ID="lblOrderID" runat="server" Text="OrderID"></asp:Label>
                        <asp:TextBox ID="txtOrderID" runat="server" CssClass="form-control"></asp:TextBox>
                    </div>
                </div>
                <div class="modal-footer">
                    <asp:Button ID="btnSave" runat="server" Text="Pay" OnClick="btnSave_Click" CssClass="btn btn-info" />
                    <button type="button" class="btn btn-info" data-dismiss="modal">Close</button>
                </div>
            </div>
        </div>
        <script type='text/javascript'>
            function openModal() {
                $('[id*=myModal]').modal('show');
            }
        </script>
    </div>
</div>

Namespaces

C#

using System.Configuration;
using System.Data;
using System.Data.SqlClient;

VB.Net

Imports System.Configuration
Imports System.Data
Imports System.Data.SqlClient

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        if (!IsPostBack)
        {
            gvCustomers.DataSource = GetData("SELECT TOP 2 * FROM Customers");
            gvCustomers.DataBind();
        }
    }
}

protected void OnRowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        string customerId = gvCustomers.DataKeys[e.Row.RowIndex].Value.ToString();
        GridView gvOrders = e.Row.FindControl("gvOrders") as GridView;
        gvOrders.DataSource = GetData(string.Format("SELECT TOP 2 * FROM Orders WHERE CustomerID='{0}'", customerId));
        gvOrders.DataBind();
    }
}

private static DataTable GetData(string query)
{
    string strConnString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(strConnString))
    {
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = query;
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.Connection = con;
                sda.SelectCommand = cmd;
                DataTable dt = new DataTable();
                sda.Fill(dt);
                return dt;
            }
        }
    }
}

protected void Display(object sender, EventArgs e)
{
    GridViewRow rowOrder = (sender as LinkButton).NamingContainer as GridViewRow;
    txtOrderID.Text = (rowOrder.FindControl("lblOrderId") as Label).Text;
    ClientScript.RegisterStartupScript(this.GetType(), "Pop", "openModal();", true);
}

protected void btnSave_Click(object sender, EventArgs e)
{
    //Your Saving code.
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not IsPostBack Then
        If Not IsPostBack Then
            gvCustomers.DataSource = GetData("SELECT TOP 2 * FROM Customers")
            gvCustomers.DataBind()
        End If
    End If
End Sub

Protected Sub OnRowDataBound(ByVal sender As Object, ByVal e As GridViewRowEventArgs)
    If e.Row.RowType = DataControlRowType.DataRow Then
        Dim customerId As String = gvCustomers.DataKeys(e.Row.RowIndex).Value.ToString()
        Dim gvOrders As GridView = TryCast(e.Row.FindControl("gvOrders"), GridView)
        gvOrders.DataSource = GetData(String.Format("SELECT TOP 2 * FROM Orders WHERE CustomerID='{0}'", customerId))
        gvOrders.DataBind()
    End If
End Sub

Private Shared Function GetData(ByVal query As String) As DataTable
    Dim strConnString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As SqlConnection = New SqlConnection(strConnString)
        Using cmd As SqlCommand = New SqlCommand()
            cmd.CommandText = query
            Using sda As SqlDataAdapter = New SqlDataAdapter()
                cmd.Connection = con
                sda.SelectCommand = cmd
                Dim dt As DataTable = New DataTable()
                sda.Fill(dt)
                Return dt
            End Using
        End Using
    End Using
End Function

Protected Sub Display(ByVal sender As Object, ByVal e As EventArgs)
    Dim rowOrder As GridViewRow = TryCast((TryCast(sender, LinkButton)).NamingContainer, GridViewRow)
    txtOrderID.Text = TryCast(rowOrder.FindControl("lblOrderId"), Label).Text
    ClientScript.RegisterStartupScript(Me.GetType(), "Pop", "openModal();", True)
End Sub

Protected Sub btnSave_Click(ByVal sender As Object, ByVal e As EventArgs)
End Sub

Screenshot