Lost all of my iTunes music after El Capitan upgrade
I have around 40,000 songs in my iTunes, almost all music ripped from CDs over the course of 15 years. Just went to listen to some on my computer and found that, although all the songs appear in my iTunes window, not one of the files can be found. I navigated to my iTunes folder where all my music files are stored, and THEY HAVE ALL DISAPPEARED. I have always synced all my music folders to Dropbox and don't use Apple's Cloud music service (because it mucked up music on my iPhone), but all the files have gone from Dropbox too. Then I realised this is the first time I have tried to listen to music on iTunes since upgrading to El Capitan. So, it seems THE UPGRADE TO EL CAPITAN HAS DELETED ALL OF MY MUSIC FILES, so Dropbox found nothing to sync.
So, what can I do? I tried the 'solution' listed here: If you don't see your content after you update iTunes - Apple Support
... but this didn't solve the problem at all - it retrieved only 40 purchased songs!
I AM DESPERATE. It seems I am not the only one to have had this problem with El Capitan. What a disgrace, Apple (and that's quite apart from Calendar now working very slowly since the update too).
Please can someone help?
MacBook Pro, OS X El Capitan (10.11.1)