Tancamb Said:
"Tunes library mostly disappeared: The migration assistant link takes me to PC - Mac instructions.: I tried to get back what was there yesterday from Time Machine, but I still am missing thousands of songs. My personal property."
Three Thoughts:
Look in other locations (i.e. USB Flash Drive). If they are not missing then Rey reinstalling the macOS.
Hold Down the Shift Key upon boot. Once at the login screen, release shift. See if it all now works. In Safe Mode, only necessary items load (i.e. the caches), and new preferences are set aside.
If neither of the above works...
Reinstalling the macOS will place moved items back to where they should be, and will replace damaged system files.
1. First, Back up your Mac:
Create a Time Machine backup of your Mac, so that you can have something to restore your Mac from, should anything go wrong with the reinstall. Use an external Hard Drive of at least 1.5 times the size of your internal hard drive.
2. Then, Reinstall the macOS:
Nothing would be deleted. It's just that items that have become corrupted or deleted, would be replaced or reinstalled back to where they should be. Go Here: How to reinstall macOS from macOS Recovery - Apple Support