So as a tech aficionado I do preach making sure that your computer is updated, including individual programs and also making sure you have some type of antivirus/security program to protect your computer. Recently in the midst of my computer cleanup and updating I finally ran the update to iTunes 11.0.2, and aside from the interface change that I wasn't fond of, it seemed fine. But a few days later I went to import some new music and noticed that my entire iTunes library was gone!!!!!!!!!!!! The horror!!!!!
So of course I complained for a few minutes and then spent the rest of the night trying to figure out why it happened as I didn't do anything other than upgrade to last version of iTunes. After some internet forum trolling I found out that the latest version of iTunes and the latest incarnation of AVG Free 2013 don't play well together. The resident shield option, which essentially realtime scans the computer and is the core of the Antivirus program was somehow causing the backup of iTunes to fail and thus when this happens the operation is halted and iTunes starts from scratch with no information unless you have all of your songs, apps, and other information backed up to iCloud.
A few modifications may have helped but the better solution was to remove AVG from my system. In regards to the missing iTunes library, I found a older backup file before I upgraded the iTunes and moved it to the iTunes system folder and everything was back to normal without me having to reimport all of my music. Now of course I couldn't be without AV protection so I installed AVAST! antivirus, it's still free and does the same job if not better. But most importantly it doesn't nuke my iTunes library! Feel free to comment if you have questions or if this has happened to you. To download AVAST: http://www.avast.com/