I have seen a bunch of people with a similar problem and have tried all the solutions I have seen, and nothing. My catalog still lists the files and the pics can be seen inside, but they are grayed out and have the '?'. I have clicked to find them and nothing. I have looked into the folder where they should be and only my most recent import. I have searched my computer for file type (raw .CR2) and have only found my archived files on my external hard drive. I have also opened LR as administrator. I'm not sure what else I can do or where else I can look but I cannot find any of the pictures I have in my catalog for this year. Even when I open a back-up catalog the files cannot be found and are grayed out with ?
The backup catalog is looking for the images to be in the same place. As you know, the images are not "in" Lightroom. Did you move the images? Was this a "clean" install of Windows 10? I did an upgrade installation, and all my images remained where they were supposed to be. If you can restore the images from your backup hard drive, and place them in the right folders, that could be a solution. A lot of work, I know. Why do you think the images are not in the folders where they were before you upgraded?
It was an upgrade to windows 10. No other files (that I have noticed) are not where they were. However, I have not been able to find the files anywhere and I did not move them.