What is the exact word-for-word message that indicates your catalog is not valid?
I'm guessing you may have accidentally opened the wrong catalog file; please go to the menu command File->Open Recent and select each catalog listed there to see if it is the one you want.
Maybe I haven't understood your question properly, but I'm wondering if you are using Import from from another catalog when you should be using the Import button, bottom left in the Lightroom window.
Lightroom is not a file browser that you can point to a folder and see its contents - images have to be imported to Lightroom, and a folder that you haven't imported anything from will not be visible in the Folders panel.
My guess is that you are trying to import an image using Import from another catalog, which can produce that error message.
The Lightroom catalog is a database that contains references to all the photos you import, and i also contains all the edits that you do in Lightroom, as well as metadata you might add to them.
To import images, simply click the Import button, and browse to the folder you want to import from.
I got another reply which told me to go to Import at the bottom of the screen and select the appropriate folder. I'm happy to say this worked. Many thanks anyway.
Many thanks for pointing out the import button at the bottom of the screen. I had completely missed this. Problem solved! Thanks again.
I've just replied to the query on the forum. Your answer pointing out the
import button at the bottom of the screen (which I had completely missed)
solved my problem. You might guess I'm a new user and finding that the
program works in strange ways but i'm sure I'll get there. Thanks again.