I had to reinstall the windows 10 operating system and backed up my photos to an external drive first
when I reimported them, half of them are fine but the other half appear to be previews that, when I open them, are out of focus and say 'photo not found"
how do I get them to be located so they will again be in focus and useable?
Did you actually Back Up your images or just the LR Catalog file?
Have you used the File Manager, File Explorer, to verify your actual images are on your external drive?
What steps did you take to perform the backup? What steps did you take to recover from the backup?