Q: Missing library content

After the last update my iTunes account wasn't working properly, items not being transferred to my phone even if they were selected and previously on devices. Music that wasn't selected was transferred etc.

I went to the Apple store and was told to delete iTunes from my computer and reinstall and that should "correct" the download error that was causing the other problems.

I deleted my iTunes account from my computer and downloaded the newest version again, only to have it load missing 75 - 80% of my library. I had over 5300 songs, dozens of music videos, digital movies, over 50 books and 30 Audiobooks etc.

Now I have 4 audiobooks, 1 book, about 25 movies and less than 300 songs.

How do I recover my library??

I've returned to the Apple store and they are useless. Saying everything from I didn't download iTunes properly to I need to refresh my system, I need to delete iTunes again and reinstall, to I sent everything to the cloud....

Can anyone assist me, please? And Thanks!