This is not a problem - at least not if you still know your registered e-mail address and your password, because you can also log in with these data, and then you'll see your user name.
If you don't even know your e-mail address, we can help you only if you were a verified member. Then you can write to us by postal mail, describe your problem and identify yourself with copies of appropriate documents - we will then check your details and try to figure out the missing information about your account (for free of course).
Alternatively, you have to re-register with us - after some time of inactivity the possibly still existing account will be blocked and deleted automatically.