My Lightroom was working just fine. I was actually in the middle of importing photos when all of a sudden it froze and crashed with an error message that said something about the catalog. I don't remember exactly what it said that time and if it was the same error message. It closed and now when I try to open it, it does not open but instead says "Unexpected error opening catalog". The options are Try Again (doesn't work), Quit (not what I want) or Choose a Different Catalog. When I try choose a different catalog, it gives a couple of options but that doesn't work either.
Sorry that you're having trouble using your catalog in Ligjtroom. Could you please let us know which version of Lightroom are you using and what is your operating system?
If you're a Windows user, please take a look at this article and let us know if that helps:
I have the same problem. I've just downloaded Classic CC to use on macOS High Sierra and get the same message when I try to open it. I have 5 other CC apps I use, yet this is the only one I've had issues with when downloading. Any advice?