Please enter the email address associated with your User account. Your username will be emailed to the email address on file.

What to do when you lost password or username?

Well. Do not panic! look just above this message, and there 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.)