Synced iPhone with new Macbook & majority of my music is gone?

It asked if I wanted to erase data from my old iTunes & sync with the new one, I said yes. Then 500 songs disappeared from my library. Not all of the them are purchased. There's still songs that i downloaded so I'm really confused as to what happened & how i can get them back!

To re-download purchased music…
- On the Mac, open iTunes.
- Sign in, if it isn't already signed in, with the same Apple ID you used to buy the music.
- Click "Purchased". The location varies with the version of itunes, but start by looking just above "Genius" under Playlists.
- Download all your purchases.

Apple has this crazy idea that all your music is gotten by the Mac. Then you sync that to the iPhone. If you want to reverse that process, try one of the third-party apps at the links below.

  • Transfer iPhone music/photos/apps onto new Macbook Pro? I just purchased a Macbook Pro yesterday and I plugged my iPhone 5 into it and was expecting the music to be automatically transferred to my computer, but that did not happen. How do I transfer my music/photos/apps onto my macbook? The computer I had previously used for iTunes is gone and unable to use.
  • New iphone and syncing to a used macbook (which old iphone was synced onto)? I'm thinking about getting the new iphone 6 but id be syncing it to my macbook pro, i'm just wondering if i would lose my photos i have stored on there, or would they sync onto my new phone, and can i stop this? I'm giving my 5s to my mum and ill be deleting the photos and everything on there so itll be new for her to use. Can someone who's good with this sort of thing help me out?!
  • How to transfer my iphone music to a new macbook? I got a macbook today and I need to put all of my music into this iTunes's library. I have all of the music I need right on my iPhone but I won't have my original mac with me until next week. Is there anyway to get this music onto my new mac without using the first computer?
  • How to transfer music on my iPhone to my new Macbook Pro? Before I got my new Macbook I had my iPhone synced up to my Parents' Mac desktop computer. Is there a way to get all of my music, apps, and movies from my iPhone all on my laptop?