LR Catalog cannot be opened in a changed user account

When you create a file in your user's profile (the active usre's file area), the files (and folders) are automatically restricted to be accessed by this user only. In this protection, the user is identify by it's internal ID, not by its display name. When you reinstall the machine and create a user, it will get a new internal ID, even if you create it with the same display name. Therefore, for the protection of the file it's a different user and it is not allowed to access the files/folders.

The way to fix this is to run Windows Explorer with admin privilege at take the ownership of the files/folders.

And, to avoid this problem in the future, to not store tour files in the active user's flie area as said by richardplondon.

The Catalog file itself may have inappropriate rights assigned somehow - in order to rectify which, an admin level login may be needed - or, to temporarily elevate your file manager with "run as administrator". Let's say, though, that this does not help: one way to still retrieve your image library might be to create a completely fresh Catalog, and then to merge in all the contents / edits / collections etc from your prior Catalog via "Import From Another Catalog". This operation does not require the ability to open and modify that other Catalog. The new Catalog will then refer to all the same source files, at the same file paths, as the old one did. It's not perfect since some matters such as Publish services would then need to be remade. So getting your old Catalog working is best..

Hereafter, I advise using a non-useraccount-specific, explicit file location for both your images and your Catalog. Not some quasi-virtual address such as "Pictures". Make a new top folder inside a given drive ("D:\data", say) and then move your image library etc bodily inside there, re-addressing its top level folder path for LR as needed.

Lightroom will then become user-account-neutral EXCEPT that various LR customisations and central user resource files are stored within the active user's files area. User presets of all kinds, watermark graphics, downloaded lens profiles... unless you previously backed those up, those may now be hard to get at. An admin user should be able to retrieve them - unless you opted for all user files to be deleted, at the time when that user account was removed.

So some kind of regular backup of all this stuff is a good idea IMO, as well as of your image files and your Catalog.

On Windows, "C:\users\[username]\AppData\Adobe" has the two subfolders of interest: "CameraRaw" and "Lightroom".

Please quote the EXACT word-for-word error message. Any Lightroom 6 in any user account ought to be able to open catalogs from any other use account.