To login you need to fill in your username or your email address, and your password.
What to do when you lost password or username?
Well. Do not panic! Just go to the login page. Scroll to the bottom of the page. Now you see two questions:
Forgot your password?
Forgot your username?
Clicking either one of these will get you to a form that will provide solutions for these problems. (You will be sent emails with the necessary login data. All this works automatically, so you will not bother anyone else; no one will even know that you lost these data.)