iTunes library deleted after iTunes update?

I recently got a new computer and I managed to transfer all my itunes files from my old computer to the new one (through external hard drive). It took a while and it's been working fine for a few weeks now.

However, 2 days ago I downloaded the new itunes update and for some reason everything in my library was deleted? There were no songs and all the playlists were gone. When I tried syncing my itunes again, it only re-downloaded songs from an old itunes library. There were only some songs but no playlists at all or ringtones.

When I connected my iphone to my computer, it recognized the phone and all my current songs/playlists/tones are still found on my phone but not on itunes. I know I'll probably have to go through the hassle of transferring everything back.

I'm just wondering if this is a common bug and if some people are having the same problem?

Also is there an easier way for me to transfer everything back into itunes again?

Thanks.