Ok, this is what worked for me, but I can't promise it will work for any of you as well.
There was something corrupting my itunes Library.itl file. I use the free version of AVG and Malwarebytes on my Win 7, 64 bit machine. I recently did an upgrade with AVG over the weekend and after everything was finished, I rebooted my computer. When I logged back in and launched iTunes, everything was gone. Seriously? How? I checked my preferences and the iTunes media folder location was set to C:\Users\Username\Music\iTunes\iTunes Media. I went ahead and reset the folder location again and left the two boxes below checked. Still, no music was there.
Fortunately, before I upgraded AVG, I did a backup of my contents to an external hard drive, which included the iTunes Library.itl file. I was able to copy over the .itl file over the new one that was created. For Win 7 users, this is located on C:\Users\Username\Music\iTunes
So if you have the .itl file backed up or stored somewhere else or even an older version of the file, copy it over the one that is present in your folder. Re-launch iTunes and the music files, playlists, etc should be there.
I have only started having this issue with the current version of iTunes (11.0.2), so it could be a problem with the program itself. However, it may just be a problem related to the computer. Whatever it is, I really hope Apple will take note of it and do some research before releasing the next version of iTunes.
This took me a little while to figure out and I know it has frustrated a lot of people, including myself.