Hi Honeyjo,

Please refer below sample.

Database

I have made use of the following table Customers with the schema as follows.

I have already inserted few records in the table.

You can download the database table SQL by clicking the download link below.

Download SQL file

HTML

<cc1:ToolkitScriptManager runat="server">
</cc1:ToolkitScriptManager>
<table>
    <tr>
        <td><asp:Label ID="lblName" runat="server"></asp:Label></td>
    </tr>
    <tr>
        <td><asp:Button Text="Show" ID="btnShow" runat="server" /></td>
    </tr>
</table>
<cc1:ModalPopupExtender ID="mpName" runat="server" PopupControlID="Panel1" TargetControlID="btnShow">
</cc1:ModalPopupExtender>
<asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center" Style="display: none">
    <table>
        <tr>
            <td>Name:</td>
            <td><asp:TextBox ID="txtName" runat="server"> </asp:TextBox></td>
        </tr>
        <tr>
            <td><asp:Button Text="Search" runat="server" ID="btnSearch" OnClick="Search" /></td>
        </tr>
    </table>
    <br />
    <asp:GridView ID="gvCustomers" runat="server" AutoGenerateColumns="false" OnRowCommand="OnRowCommand" OnSelectedIndexChanged="SelectedCustomer" DataKeyNames="CustomerId">
        <Columns>
            <asp:BoundField DataField="CustomerId" HeaderText="ID" />
            <asp:BoundField DataField="Name" HeaderText="Name" />
            <asp:BoundField DataField="Country" HeaderText="Country" />
            <asp:CommandField ShowSelectButton="True" />
        </Columns>
    </asp:GridView>
</asp:Panel>

Namespaces

C#

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

VB.Net

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

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.BindGrid(string.Empty);
    }
}

protected void Search(object sender, EventArgs e)
{
    this.BindGrid(txtName.Text.Trim());
    mpName.Show();
}

protected void SelectedCustomer(object sender, EventArgs e)
{
     string id = gvCustomers.SelectedRow.Cells[0].Text;
     string name = gvCustomers.SelectedRow.Cells[1].Text;
     mpName.Show();
}

private void BindGrid(string name)
{
    string conString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(conString))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT CustomerId, Name, Country FROM Customers WHERE Name LIKE @Name + '%' OR @Name IS NULL", con))
        {
            if (!string.IsNullOrEmpty(name))
            {
                cmd.Parameters.AddWithValue("@Name", name);
            }
            else
            {
                cmd.Parameters.AddWithValue("@Name", DBNull.Value);
            }
            con.Open();
            this.gvCustomers.DataSource = cmd.ExecuteReader();
            this.gvCustomers.DataBind();
            con.Close();
        }
    }
}

protected void OnRowCommand(object sender, GridViewCommandEventArgs e)
{
    if (e.CommandName == "Select")
    {
        lblName.Text = (gvCustomers.Rows[Convert.ToInt16(e.CommandArgument)] as GridViewRow).Cells[1].Text;
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Me.BindGrid(String.Empty)
    End If
End Sub

Protected Sub Search(ByVal sender As Object, ByVal e As EventArgs)
    Me.BindGrid(txtName.Text.Trim())
    mpName.Show()
End Sub

Protected Sub SelectedCustomer(ByVal sender As Object, ByVal e As EventArgs)
     Dim id As String = gvCustomers.SelectedRow.Cells(0).Text
     Dim name As String = gvCustomers.SelectedRow.Cells(1).Text
     mpName.Show()
End Sub

Private Sub BindGrid(ByVal name As String)
    Dim conString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As SqlConnection = New SqlConnection(conString)
        Using cmd As SqlCommand = New SqlCommand("SELECT CustomerId, Name, Country FROM Customers WHERE Name LIKE @Name + '%' OR @Name IS NULL", con)
            If Not String.IsNullOrEmpty(name) Then
                cmd.Parameters.AddWithValue("@Name", name)
            Else
                cmd.Parameters.AddWithValue("@Name", DBNull.Value)
            End If

            con.Open()
            Me.gvCustomers.DataSource = cmd.ExecuteReader()
            Me.gvCustomers.DataBind()
            con.Close()
        End Using
    End Using
End Sub

Protected Sub OnRowCommand(ByVal sender As Object, ByVal e As GridViewCommandEventArgs)
    If e.CommandName = "Select" Then
        lblName.Text = (TryCast(gvCustomers.Rows(Convert.ToInt16(e.CommandArgument)), GridViewRow)).Cells(1).Text
    End If
End Sub

Screenshot