Entire Library Has Disappeared From iTunes Match

I'm using a MacBook Pro (Retina, 13-inch, Mid 2014)

El Capitan Beta 10.11 (15A215h) with iTunes 12.2.1.16

I have an existing iTunes Match subscription, doesn't need renewing until Dec 2015 so all up to date. I am also subscribed to the Apple Music free trial just now.

My entire library of 20,000+ songs is uploaded and matched.

Last night I created a Playlist to download them all to a spare HDD. It got through 4,000 songs before quitting. However now my entire cloud library cannot be accessed, and no songs can be played. Obviously I don't have a local copy to play, and every song now has an 'error' cloud icon against it.

Where are my songs ?!