I use Windows 10 and Lightroom Classic (photos on hard drive not in the cloud.) I recently had to restore my Lightroom catalog after replacing my harddrive, but not all my images will link now. Many will, but those that won't are ones that were automatically renamed "psd" (by Lightroom) because of having some edits done to them in Photoshop before being saved in Lightroom. The names show up in the catalog with greyed out thumbnails. I can click on the little rectangle that then asks me do I want to locate the image, and when I say yes, it does open the folder of original images on hard drive for me to browse. But if I choose the original image, it tells me it's already in Lightroom and I can't add it again. (Apparently some of the images I did multiple different Photoshop versions of, and for these I can also locate other versions of the psds, when it allows me to browse to locate, but not the version with the latest title. For example, I can browse and find the psd IMGP1691-Edit-2, but not IMGP1691-Edit-5, which is the greyed out one in Lightroom and the final version I want to use.) I've never had to deal with this issue before, and am not a tech person, so I would be so grateful if anyone's able to help me figure it out! Thnx so much!!!
If it's greyed out, it's because it doesn't exist where you told Lightroom it was located. Please take note that "XYZ-Edit-2.psd" is a completely different file from "XYZ-Edit-5.psd", and if you moved or deleted the "-5" version at some point in the past, you would have had this problem regardless of the reinstall/restore.
The best bet would be to note which exact filenames are missing, and do a Windows search for that filename. See where the file lives, or if it even exists. It may be you inadvertently moved it to a new location, or maybe accidentally deleted it, but regardless, searching and (hopefully!) finding the file in Windows will help get to the root of the problem.