I don't know why but I got an update and all my music was gone from the iTunes library and I got them back and quit iTunes a few days later and all the music was gone again WTF?!
No idea. Bad RAM. Bad hard drive. Corrupt preferences. Corrupt iTunes library. iTunes library in a strange place confusing iTunes. All of the above. None of the above.
It's probably just iTunes using the wrong library. Hold option while starting iTunes and it will prompt you to select an existing library. Then navigate to the folder containing the iTunes library which holds all your music and select it.
It's probably just iTunes using the wrong library. Hold option while starting iTunes and it will prompt you to select an existing library. Then navigate to the folder containing the iTunes library which holds all your music and select it.