All downloads are tied to the account that downloaded it. Do you have any of its purchases on your computer's iTunes library : http://support.apple.com/kb/HT1920 - or on a backup that you can try adding to your library via File > Add To Library so that you can get its id ?
If not then see if you can get the id (and reset its password) via http://iforgot.apple.com
Whether you can redownload music for free (e.g. via the Purchased tab in the iTunes store app on your iPhone/iPod Touch, or the Purchased link under Quicklinks on the right-hand siden of teh iTunes store homepage on your computer's iTunes) depends upon what country that you are in (music can't be redownloaded in all), whether you've changed countries since buying it, and if it's still in your country's store