I downloaded one image from my (now missing catalogue) and then LR crashed. When I reopened LR, it asked me to upgrade to the most recent version, which I did. I now cannot find this cataloge anymore! When I open "recent" it isn't there. Any ideas?
A word of note: I'm still trying to understand catalogues and would welcome any insight - or links to understanding - on how they work.
The crash probably whiped out the 'recent catalogs' menu, so you just have to find that catalog manually. That should not be a big deal. The default place for the Lightroom catalog is your Pictures folder. Go throught it an look for that catalog. The catalog file has a .lrcat extension. That may help finding it.
The Lightroom catalog is literally the only file Lightroom Classic opens. When you import images you are adding references to where your images are located on your computer as records in the Lightroom catalog. You are not adding the images to the catalog. As you make adjustments to your images, those adjustments are stored in the catalog. They are NOT applied to the image files. When you create collections those collections are groupings that are stored in the catalog. When you create keywords those keywords are stored in the catalog. The catalog is the center of operations in Lightroom Classic. It is essential that the catalog is backed up on a regular basis. It is also essential that you have some method of backing up your images because they are not backed up by Lightroom. You need to use other software or backup services to do that job if the images are important to you.
The catalog is a database. As such it is a powerful tool that, if used properly, can enable the user to categorize images to be searched using keywords. Images can be grouped into collections by subject as desired, making them easy to locate. But managing a database is not an effortless, automatic thing. A good database requires good management and constant attention to be used effectively. And how good your catalog works for you is up to you and how well you want to maintain it.
As far as how often to back up is concerned, I suppose that depends on how active you are. If you are a daily photographer and do a lot of editing then it would probably be wise to configure Lightroom to back up on a daily basis. I don't shoot that often, so I have Lightroom configured to back up on a weekly basis and I'm comfortable with that. Occasionally I go to my backup folder and remove old backups, only keeping a handful of old backups. My computer system has proven to be quite stable. It has only been necessary for me to rely on backups a couple of times over the last 10 years.
This is a general overview of catalogs. Probably isn't everything you want to know. Feel free to ask more. You might be able to find a tutorial on YouTube you can do some searching that might provide more information. There are so many tutorials there that I didn't really do a search for you.