Hi:
Have you changed the Login page under "Admin / Site Settings" ?
If you set a page under login and this page does not have "All Users" view permission the result is a neverending redirection when you try to log in.
If this is your case, here you have the solution:
http://blogs.snapsis.com/PermaLink,guid,3078311e-6925-4c03-8243-d1b9c8a11906.aspx
Regards,
Leandro Montedoro