Page.User.Identity.Name returns null

Hi,
I am using ASP.NET 3.5 with Visual Studio 2008. I am a new bee in ASP.NET and working on a website where it can have 3 types of member visiting to it like admin, registered users and annonymous users. I want to provide authentication and session management for these users.
I am not using Login Control provided MS/.NET rather created my own. Below is the code I am using.:
// Check authentication. Guid returned should not be same as 0000-0000-0000-0000
            string token = authenticator.AuthenticateUser(UserName.Text, Password.Text); // This //method uses Membership class to validate user. On successful validation, it returns //FormsAuthenticationTicket to UI.

            if ((token == null) || (token.Length == 0))
            {
        ...