iTunes library disappeared after factory reset of iMac

Hello, Cheapster.

Thank you for using Apple Support Communities.

Reading your post, I understand you're missing your iTunes library after restoring your Mac to factory settings. I'd like to help. If you're an Apple Music subscriber, make sure that you sign in with your Apple ID and you turn on Sync Music: Turn on Sync Library with Apple Music

If this was purchased music, you can redownload them using these steps: Redownload apps, music, movies, TV shows, and books

If this was music you've ripped from CDs or obtained from other resources, then you'll need to copy the music from your backup and add it to your library: Import music from a computer to Music on Mac

Let me know if this helps.

Cheers!

Hello, Cheapster.

Thanks for getting back to us here in Communities. For the issue with iTunes session expired, try deauthorizing your Mac and then authorize it: Authorize or deauthorize your computer for iTunes Store purchases

If that doesn't help, contact Apple Support.

Cheers!