I've been having my Library disappear frequently since the latest Apple update. I restore the Library from my backup files, only to find them gone again later in the same day. Yes, it's easy enough to add the back up files back into the Library....but not if you have to do it several times a day!
I've uninstalled Itunes at least 3 times by now, hoping to eliminate the problem. Unfortunately, in the process I eliminated the older version, so I can't just revert to the one that actually functioned! But nobody should have to revert to the older version. APPLE NEEDS TO FIX THE BUG IN THIS VERSION! It's clear from the forums that numerous people are having the same problem.
I don't believe the problem lies with AVG, as many people have speculated. Listing Itunes as an Exception does NOT work. And I'm sorry, but one program (Apple's) should not force me to give up another good one (AVG), especially when it's protecting my computer!
Long story short: I managed to keep my Library intact for three days running after I unchecked the box in Preferences (General tab) that updates Itunes automatically. To test my suspicions, I checked the box just a few minutes ago, and the whole Library INSTANTANEOUSLY disappeared again.
IT IS THE APPLE UPDATE THAT IS REMOVING THE LIBRARY. IT IS THE APPLE UPDATE THAT NEEDS A FIX. Please stop blaming AVG.