Have you registered you login information? I found it was in an unusual place in Joomla- which I will be looking for again as I have a strange login- related issue to unravel.
I am wondering if this could be the source of my problem. My webserver installed Joomla and sent the login information to me, which did not work. Then I found a place to register on Joomla, but not the home page ( I can't locate that now). Is it possible that the registration of the admin login is supposed to occur on my webserver's end and that when I registered on Joomla it caused the front end and back end logins to become entangled with each other?