Ya that just happened to me this morning, and I started to dispair. I found Calibre empty, and an emply "Library" folder on my desktop.
Then I remembered a few days ago, when I was trying to clear up my desktop, I moved my full "Library" folder from my desktop into another folder. I didn't think this would be a problem because this "Library" was a folder I made for Adobe Digial Editions, before I downloaded Calibre.
Then I remembered that when I installed Calibre I put Calibre's own files location folder, also generically named, "Library", into this old "Library" folder with all my other ebooks. So this is where Calibre was reading my books from.
Then I realised that instantly, as I opened the program for the first time after moving my original "Library" to a new location, Calibre had created a NEW, empty folder generically named "Library". I guess Calibre couldn't find where I had replaced my original "Library" and created a new one on my desktop. So I went to where I had placed my original "Library", and sure enough, all my books were in there.
So I simply closed Calibre, deleted the new, emply "Library", and replaced the original "Library" on my desktop, where it had been before. Then I opened Calibre again, and my ebooks and collections were restored
. I wonder if you might have moved your library folder, or perhaps someone else moved it without your knowledge. Hope this helps.