WHAT HAPPENED TO ALL MY MUSIC. Even the previous libraries have been reduced to 1/4 or 1/5 of the size of my library.

I downloaded 10.0.02 and most of my music disappeared. I had already lost the ability to sync my phone with my laptop. Thought the updated software might solve the problem---but that's when the nightmare started. Then, needing a new phone anyway (and thinking THAT might solve the problem), I bought the iPhone SE (64 GB -- old phone 5S was only 32 GB and was almost full). That made everything worse. When I go to "Previous iTunes Libraries" on my computer, they are ALL missing most of my music. What happened??? How can I fix it?