I have been in Lightroom Classic purgatory since October 25 when all of a sudden an upgrade was required and my 2011 iMac didn't meet system requirements. The time for a new computer had come. After weeks of migration issues to the new iMac (Catalina OS), installing the latest version of LR Classic, and three visits by a computer guy to get all the files in the right place, finally it looked like all was well. When I restarted the computer I got the message that "catalog upgrade" was necessary. This process has been going on for at least five days, very very slowly. Thousands of photos have the ! box meaning the photos are missing. I have paused and resumed the sync several times, quit and reopened LR, and sometimes the numbers go up a bit on All Synced Photos but now it's just stuck doing nothing. Also, the number of All Photographs is about 10,000 short. What should I do to at last be able to use Lightroom again?
No, instead of opening LrC by clicking/double clicking on the shortcut to start, and accepting whatever option you left it in as to what catalog to open (default probably most previous) use your MACOS Finder to navigate to the catalog(s), looking for the one you think LrC should have opened. Double click on that catalog in Finder as to start LrC using that specific catalog.