Figuring out asp.net code that I did not write.

Hello Experts

I have recently been asked to work on a website that was programmed a couple of months ago but abandoned mid way. I have spent many hours trying to make sense of this website and how it actually works but I am stumped. I am including a snippet of the login page below. Any information on how this webpage actually processes the login would be greatly appreciated. I also believe that the website might also be utilizing web services but I am not certain about that and have never worked with web services before. Also please note that I cannot actually find the code behind file Login.aspx.cs which is puzzling me the most. How does the server process the login button click when there is no .cs file?? Any insight you could provide would be greatly appreciated.
Thanks.


<%@ Page Language="C#" MasterPageFile="~/MasterPages/Default.Master" AutoEventWireup="true"
    Codebehind="Login.aspx.cs" Inherits="Web_Client.Login" Title="Login" %>



<asp:Login ID="ClientLogin" runat="server" OnAuthenticate="ClientLogin_Authenticate"
            DestinationPageUrl="~/Default.aspx" DisplayRememberMe="False" PasswordRecoveryText="Forgot Password" PasswordRecoveryUrl="~/Registration/ForgotPassword.aspx">
            <LayoutTemplate>
                <table border="0" cellpadding="1" cellspacing="0" style="border-collapse: collapse">
                    <tr>
                        <td>
                            <table border="0" cellpadding="0">
                                <tr><td></td>
                                    <td align="left" >
                                        Log In For Returning User</td><td></td>
                                </tr>
                               
                                <tr>
                                    <td align="right">
                                        <asp:Label ID="UserNameLabel" runat="server" AssociatedControlID="UserName">User Name:</asp:Label></td>
                                    <td>
                                        <asp:TextBox ID="UserName" runat="server" ></asp:TextBox>
                                        <asp:RequiredFieldValidator ID="UserNameRequired" runat="server" ControlToValidate="UserName"
                                            ErrorMessage="User Name is required." ToolTip="User Name is required." ValidationGroup="ClientLogin">*</asp:RequiredFieldValidator>
                                    </td>
                                    <td></td>
                                </tr>
                                <tr align="left">
                                    <td align="right">
                                        <asp:Label ID="PasswordLabel" runat="server" AssociatedControlID="Password">Password:</asp:Label></td>
                                    <td>
                                        <asp:TextBox ID="Password" runat="server" TextMode="Password" Width="151px"></asp:TextBox>
                                        <asp:RequiredFieldValidator ID="PasswordRequired" runat="server" ControlToValidate="Password"
                                            ErrorMessage="Password is required." ToolTip="Password is required." ValidationGroup="ClientLogin">*</asp:RequiredFieldValidator>
                                    </td>
                                    <td><asp:HyperLink ID="PasswordRecoveryLink" runat="server" NavigateUrl="~/Registration/ForgotPassword.aspx" TabIndex="3">Forgot My Password</asp:HyperLink></td>
                                </tr>
                                <tr>
                                    <td align="center" colspan="3" style="color: red">
                                        <asp:Literal ID="FailureText" runat="server" EnableViewState="False"></asp:Literal>
                                    </td>
                                </tr>
                                <tr>
                                    <td align="center" colspan="3">
                                        <asp:Button ID="LoginButton" runat="server" CommandName="Login" Text="Log In" ValidationGroup="ClientLogin" OnClick="LoginButton_Click1" TabIndex="2" />
                                    </td>
                                 
                                </tr>

                            </table>
                        </td>
                    </tr>
                </table>
            </LayoutTemplate>
        </asp:Login>