Despite the terse phrasing it kind of is the answer. If you don't have a backup you will have to get it from the original sources again and rebuild your library organization as best you remember.
If it was purchased from the iTunes Store and your country permits you may be able to redownload it.
Downloading (using iOS or computer) past purchases from the App Store, iBookstore, and iTunes Store - http://support.apple.com/kb/ht2519 - enabled with iTunes 10.3 and newer; not all media formats are available in all countries (see: iTunes in the Cloud, iTunes Match, and iTunes Radio availability by country - http://support.apple.com/kb/HT5085); apps, books (not audiobooks), music, t.v. shows, and movies (some - not all studios have permitted this). Downloading previously purchased movies and TV shows requires iTunes 10.6 or later. Discontinued items not available. For items not included in the iCloud list (e.g., ringtones, audiobooks), or locations or computer systems where iCloud is not (yet?) available, you only get one download per fee paid. Apple recommends, "You always back up your iTunes library in the event that a purchased item is no longer available on the iTunes Store," (http://support.apple.com/kb/ht2519).
If it was from CD you will have to re-rip the CDs.
Other sources, whatever they are...
I guess if you are very, very lucky the files may still be there but the library file was lost, or perhaps the locations broken. You don't say exactly what you are finding when you look in your iTunes folder or when you open iTunes itself.