Button click event not getting fired!!

I have a button inside gridview.I have written click event for the button.but its not getting fired

 <asp:Button ID="btn_Book" runat="server"   OnClick ="btn_Book_Click" Text="Book" BackColor="#990000" ForeColor="White" PostBackUrl="ReservationPage.aspx" />

 protected void btn_Book_Click(object sender, EventArgs e)
    {
        string i = GridView1.SelectedDataKey.ToString();
     
    }