How do I find the account a steam game belongs to?

I own a hard copy of Left 4 Dead but unfortunately it is registered to my old Steam account, which I made several years ago and have since forgotten the password and even the username. Is there a way to enter the CD key and find the account it is attached to?

Solution 1:

The only way would be through the email address associated with the account. A reverse-engineering from the CD key to any account name is not possible.

Solution 2:

uncovery's answer is not actually true: there's a legit way to discover, which account your CD-Key belongs to by contacting Steam Support.

You need to mention that you have forgotten your account name and the only proof you have is the CD-Key. Be honest with them and they'll definitely help you.