You need to add some code to login page so here are the necessary steps:
1. First you need to retrieve your username and password from session on Login page. You can add all the code in Login.aspx .cs. to it's load method.
2. Use below code snippet to validate user
Membership.ValidateUser(username, password);
3. Then use this code snippet:
FormsAuthentication.SetAuthCookie(username,
true
);
4. And redirect user:
ctlLogin.RedirectAfterLogin();
Regards