Q: two songs downloaded can not be used because original files can not be found. how do you restore these files

Maybe the files aren't really missing.  Did you look in the series of folders in the iTunes media folder?  If they are there then when iTunes asked you to locate them do so.  If they aren't there you will have to restore them from wherever you got them from in the first place.  I presume you downloaded them from the iTunes Store, though you don't say.

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).