I was able to login with the same credentials yesterday but I get an invalid username error message today.
That said, I think the issue is caused by me using the same email address for two accounts. The first account was created on WordPress.com the other account was created while finishing the WordPress installation (step 5) http://codex.wordpress.org/File:install-step5.png
I changed the email address linked to the account created on the wordpress website today. How can I retrieve the login information for the other account.