Q: Why are thumbnails in Photo are suddenly gone?

I opened Photo this morning on my mid-2014 Macbook Pro (Retina, 15in) with Yosemite and all my thumbnails were gone on over 11,000 photos!  Most of the photos appear to be there. I searched for answers and tried the "duplicate" solution (you get a "version 2" and then delete the old one) and also the batch "rotate clockwise and then counterclockwise", both of which seem to bring the thumbnails photos back.  I spent 7 hours today rotating photos in small batches and only got through a few thousand.  I closed Photo (as I had several times today) to take a break and when I opened it up again, ALL the previous edits were gone and it was just like it was early this morning. 

I use my photos in professional development with teachers and at informal science presentations and need an intact system.  I have NEVER, in all my years of using Mac (since 1980...) seen something as puzzling and devastating happen spontaneously to one of my computers.  Is there malware or a virus going around?  I keep my technology clean with Sophos.  My Macbook Air (early 2015) with Yosemite is fine, as is my brand-new iMac with El Capitan.  Should I replace the Photo library with a copy I saved to import into my new iMac?  Should I upgrade to El Capitan?  There are so many negatives online about the newest OS softwrae that I'm not sure whether it would make the problem worse.  Help, please!!!