After updating to 11.3.1 all of my library is gone

Question

Yesterday, I finally updated my iTunes to the latest version 11.3.1.2.
Now my entire library is missing from iTunes.  There isn't even a Previous libraries folder in the /users/user_name/my music/itunes folder.  I still have all of my local music files, but my playlists and everything is completely gone.
I tried searching for other *.itl files, but found none from anything recent except one from a manual backup I completed of my documents folder back in '12.
My questions are:
How did these files disappear?
Why did iTunes not create a proper backup prior to upgrading itself?
How do I get them back (I only sync part of my music to my devices because of the amount of movies/music/etc.)?  I know I can go and manually add them back, but I am still going to be missing my playlists.....

Answer 1

Thanks for the reply.  Unfortunately, there was no previous libraries folder as I mentioned in the post.  I was able to retrieve the old files via previous folder versions.
When I started up iTunes and noticed that there was nothing there where no cloud files showing.....no playlists, no songs..... nothing). 
I took the library file that I retrieved from the day that it updated and replaced the iTunes Library file with it.  The cloud files showed up, but the local files, videos, and playlists were still missing.
The local files and videos aren't such a big issue, and I am sure that when iTunes syncs itself with my other devices, the playlists will re-import (maybe I am wrong here). 
I wound up just copying all the files back over, and still the same was there after restarting iTunes.  So far, I have reimported all of the local files.... 
Just really frustrating that this happened.  I am seriously frustrated because of the amount of time that I have spent trying to fix it. 
I would really like to understand why this happens.  And I would recommend that anyone else that even has iTunes, immediately make a personal backup of their .itl files and/or make sure that they have the system feature enabled on their drive to make sure they have access to older versions of the folder.