Q: how to get itunes files back

The easiest way to get back missing items?  Restore your whole hard drive with all your irreplaceable things like photos in about 20 minutes from the backup you make on a regular basis.

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,  "... back up your iTunes library. " (http://support.apple.com/kb/ht2519).

"After you download your purchases, back up your iTunes library. Previous purchases may be unavailable if they've been refunded or aren't on the iTunes Store." - http://support.apple.com/kb/ht2519

Additional tip at https://discussions.apple.com/message/24176756#24176756

  Select the iTunes Store on the left side of iTunes if you have the sidebar displayed, or with the button at the top right.

  Click on Purchased on the right side under Quick Links (see https://discussions.apple.com/message/26364186#26364186)

  You can re-download your available previous purchases.