Could you please let me know if you are able to open a empty catalog.
Also try another backup catalog and see if you are able to open it or not.
Will not do anything.
Cannot create a new catalog or open an existing one
Could you please attach a screen shot of the error that you are getting.
Try to start lightroom.exe directly from its folder and hold the Ctrl key while starting - this opens a dialog where you can select a catalog or create a new one before LR actually fully launches.
Please try creating an empty catalog as suggest by F.McLion once and let me know if it works.
The only thing that worked was to completely reinstall Lightroom 6 and then apply the latest update.
After that I was able to use the last catalog that had been backed up.
You guys seriously need better support than this forum if you want to consider yourself a professional use product.
When all else fails reinstall, but this kind of support is not what you need.
Capture One becomes a better alternative if you have a serious issue.
I am running window 10 Pro and have been having the same issues with opening Lightroom 6.5 catalog. Done a complete uninstall and reinstall of lightroom and applied 6.5 updated. It worked twice and NOW back to the very start. Can't load any catalog even the backups. I have tried to change the permissions. But, can't find the library files in windows 10. This state of affairs is very sad as this software is marketed for photographic professionals. Who don't have time to wast in debugging this type of software. Adobe shouldn't have released this upgrade with these issues in the first place.