How many usernames do you have?
Do you log out and clear your browser cache and cookies before logging in with another username?
Which username is the blog you referred to registered under?
Is it the exact same username account who registered the blog and is the Admin of the blog in question?
When you register a username account here all that's required is an email address, which becomes your unique identifier. If you are logged in and register a blog then it's registered under that unique identifier.
You cannot access any blog and work on it unless you are logged in under the exact username account that registered the blog.
Clicking this link should display all your blogs registered under the same username, including the hidden ones: https://dashboard.wordpress.com/wp-admin/index.php?page=my-blogs&show=hidden
If you do not see the blog here http://en.support.wordpress.com/my-blogs/ then
follow this guide > http://en.support.wordpress.com/my-blogs/#my-blogs-dashboard-visibility