I have had exactly the same problem with an iPad air. Recently upgraded to 8.1.3. Tried to copy some new mp3 songs to it via the iTunes software running on my Windows 7 64 bit PC. After copying on a few tracks ALL THE SONGS ON THE IPAD DISAPPEARED except for the U2 album which had been automatically down loaded by Apple. However, the itunes page on the PC still showed them as being on the ipad!
Talked to Apple support. They suggested running a diagnostic test (which I downloaded using their instructions). Emailed it to them - ipad ok but think that somehow one of the applications might have been corrupted.
At Apples suggestion, did a factory reset ( wipes all user data!)
Still exactly the same problem, Managed to re copy 63 tracks back on to the Ipad using itunes software again and then the last copy seem to go wrong and ALL OFF THE 63 TRACKS DISAPPEARED.
Apple also suggested that if this did not work - I should do a Restore via the itune software using their server. I did this - took over an hour to down load all of the operating system and the apps etc. Install went ok.
Tried Copying again - seemed to be working BUT AFTER 34 TRACKS WERE COPIED EXACTLY THE SAME PROBLEM OCCURRED. ALL THE 34 TRACKS DISAPPEARED .
Unless I have a faulty ipad (unlikely as everything else works and no error codes are shown) there must be a problem with this software release (8.1.3)