I just went thru an ordeal.....when I decided to bite the bullet and upgrade my OS from Windows 8.1 to W10 (reckoned it was inevitable, and better take the bull by the horns and getter done). Well, I backed everything up as best I knew how, purchased the upgrade SW from local best buy here in Canada, and plunged. Things went sideways from the getgo with a notification that upgrade while keeping all my files and programs was not going to work (something to do with my disc configuration...), so I went ahead with a clean install, figuring I would just have to reinstall my programs, and it was probably good for a general purge anyway. When I reinstalled Adobe CC and LR desktop version, I pointed it at my old catalogue and horrors... no previews. Oh well... let's re-import (~50,000 photos). Pointed to my photos on my data drive (seprate from my system drive, which I had to reformat as part of clean install), and let'r rip. It choked partway thru as I was running out of disc space on my SSD system disc. Started over with a new catalog on my larger data drive, and it started chugging away and I went away for a drive for a couple hours, came back to find it still chugging, left again for for a meal and alcohol solace. Came back to find it done. But NO PREVIEWS in library mode. could select a photo, even though it was just a blank box in preview, switch to develop module, and it comes up. Befuddled, I web-searched and found repeated information that it was indicative that the preview portion of my catalog was probably corrupt, I should delete or rename it, and LR would generate previews as needed. But it wouldn't. Disabled use of graphics processor -- no joy. In growing desperation, I un-installed, and reinstalled LR. Tried again, importing just a few photos to a new catalog. Same damn thing! More desperate research, and I came across a little reminder that LR uses the monitor profile to display previews. I hadn't yet enabled my monitor profile, so I did; and .....HALLELUJILLA Now pointed LR at my old catalog; > peace and relief. Posting this with the hope that it may be of use to someone else, although probably noone else is as stupid as me!