Try to read the various suggested posts dealing with the same problem (bottom right of your post):
If you can open the catalog but can't find the good one (not in the suggested list of catalogs), you'll have to search from your Explorer or Finder for files named 'catalog.pse12db'. Those database files are in catalog folders. Such catalog folders my be 'hidden' by default, so you'll have to set your Explorer to show 'hidden files and folders'. If you find one or several such folders, they are the catalogs on your computer. You can open the organizer with those catalogs by double clicking on those catalog.pse12 files.
If you can't open the organizer and get an error message about missing or bad catalog, try opening the organizer while keeping the shift key pressed. You have the options to open a new catalog or repair the selected one.
Many thanks - I was getting frustrated having spent ages onto Adobe help chat - you solved it in seconds!
Another quick question:
I am transferring my photo files (20gb +) from an old laptop to a new one and I want to preserve organiser version sets, tags and star ratings - last time I did this I lost the tags. What is the safest way? I have a dropbox facility, thought that might be best way to move big files. I've also seen mention of Adobe Cloud - sounds like an option?