mmm - weird so the site is displaying in the front end with the theme (without being renamed)? If so and you can navigate through the site what is the actual address shown in the address bar as you navigate from page to page (or post).
If you installed into a directory called 'wordpress' so you get pages at myurl/wordpress/pagename you should be able to get to it at myurl/wordpress/wp-admin.
If that doesn't help, did you add any plugins at the same time as changing the theme? If so and one is misbehaving it usually takes down the both the front & back ends, but renaming the plugins folder deactivates the plugins & gets the site back. I don't think it sounds like that is the problem in this case.
Not many clues so far to help unravel the mystery of the disappearing login. Can you post the actual URL to the site so I can have a look?