After upgrading iTunes all my media has disappeared

I have just upgraded to iTunes 12.2 on OSX Yosemite. Initially iTunes refused to open at all so I rebooted. Now I see that I have no media in my library (other than items bought from iTunes directly).

I have tried holding down the option key when opening iTunes so that I can choose my library, but that seems to make no difference. I suspect it has flattened by .ITL file during the upgrade.

I thought the days of iTunes losing all of your music were over? Apparently not!

What is the best way to fix this? Do I have to manually re-add all of my music and movie files again?

I kind of fixed it. I signed out of iTunes, then my music appeared! It seems to remain in place when I signed back in, so maybe, all is good