Sounds like Lightroom created and opened a new catalog. Your old catalog should be available from 'File - Open Recent', or else you can manually select it via 'File - Open Catalog'.
You shouldn't even need a backup. The original catalog should still be there.
It's still there but has zero pictures in it.
That does not sound like your original catalog, but a new one. Lightroom does not remove all pictures all by itself.
In the other thread about a Disappearing catalog you stated,
Add me to the list. Lightroom updated, asked to upgrade the catalog and after saying yes, I ended up with an empty catalog rather than the 60k+ photos I had originally.
The only time LR Updates/Upgrades the catalog is when you are going from an Older LR version to a Newer LR version. No LR Classic CC V7 Update, a Update from a previous Version 7 point release to a newer point release like V 7.3.1 to V 7.4, needs to Upgrade the catalog.
Then in another post in that thread you said,
raymanningphotography Jul 30, 2018 7:23 AM
I have been on LR Classic CC via subscription since around the time it came out. So I suspect it's a different bug.
That was in response to my post about a bug in the LR updater that allows LR 6, the one that has a perpetual license, to be Upgraded to LR Classic CC V7 and that would call for a catalog upgrade. Or even from LR CC 2015 (the subscription based version of LR 6) to LR 7. That too would call for a Catalog Upgrade.
If you had been using LR Classic CC since it came out and just recently did an Update for it, like stated above from V 7.3.1 to V 7.4, the catalog file would NOT of needed to be UPGRADED. The same catalog that works with the Base Version 7, the Original release of V 7, would still work with V 7.4 (At least it does on my system).
So I suspect you are dealing with some other problem and or you had been running LR CC 2015 - LR 6 and then installed LR Classic CC V7.
Please do a search of your computer for .lrcat and see how many actual catalogs you have on your hard drives. The backup for LR C CC V7 and LR CC 2015/LR 6 will end in .ZIP and need to be decompressed to work. Backups from LR 5 and older versions are just copies of the catalog file and end in the .lrcat extension.