I'm not at all sure what happened, though I received the following message when trying to import some new photos (see attached). I ended up clicking choose a different catalog and I made a new one, and now I think I have two catalogs? This is LR on my laptop where I mainly use it to use go through recently taken photos before transferring them to my desktop' s primary catalog, so I am not overly concerned with the catalog issues, however, I want to make sure it does not happen on my main computer. And, I only have one version of Lightroom installed on the laptop.
1. What happened that wouldn't let me open my main catalog?
2. Where do I switch back and forth between these two catalogs?
It happened because either your computer or LR crashed. It left behind a .lock file. Delete that .lock file and you can then open LR. That .lock file is in the same folder as your LR catalog file
1. It might be possible that you have two processes for Lightroom application or you have the catalog open in another application as well.
Please check the activity monitor if you have the above message again for the open applications.
2. To switch catalog click on File> Open recent and select the catalogs.