Did you also transfer all your images? Keeping them in the same folders they were in on the older PC and placing them in the same Top Level folder as they were on the older PC?
The Catalog file is just a Database file that holds References to where the images are actually stored.
You'll need to be a bit more descriptive about what and how you transferred your images and LR catalog to the new computer.
Windows 10 is a bit more restrictive in Folder/File permissions so If you originally store your images in the C:\Users\YourUserName\Pictures (or \Documents\Pictures) folder Windows 10 might of locked that down restricting access to it from some programs.
My images are all stored on "Drive E." The Lightroom Catalog was stored on a different drive (not "Drive C"). I retrieved the Adobe/Lightroom folder from the user/Appdata folder on Drive C to retain my presets, etc. I copied them to the appropriate folder on the new Windows 10 PC.
I have mounted the image containing drive on the new PC as "Drive E" as it was previously.
If I use the Windows File Explorer to look at the images, I see, in the case that I am highlighting, 3 folders of images. When I look in Lightroom at the folder list, I see one folder (the "first" one alphabetically by name). which appears to contain all of the images from the 3 folders. This was my first indication that something is wrong. When I checked my collections, many of them are missing.
I have not backed anything up since moving the files, since I do not want to take the chance of destroying all of my editing information. I still have a Lightroom backup from the other day, from the old PC.
I have recently purchased a Nikon D7200. The NEF files are much larger (20mb +) than those from my D90 (10mb +). I have not carefully checked the folder list from the D90, so I don't know if any of them are combined as well.
I have reviewed the link provided by Rohit, and I believe that I have followed those guidelines. The strange thing is that Lightroom sees a different folder structure than the Windows File Explorer sees.
Thanks for the info... any further suggestions?
I have attached screen captures of the Windows File Explorer panel and the Lightroom Folder panel. Note that images 1667-1973 are not loaded. The total number of images in the 3 folders is the 1864 figure shown in Lightroom.