What type of computer?
The exact version number of the OS.
The exact version of LR you are using.
Did you store the catalog file on an external drive?
I managed to find it and unzip and restore. But have no idea why it disappeared. I am funning an HP, Windows 10, latest verstion of LR. Searched for the name of the catalog because lrcat does not work it is lrdat or something like that. But unzipped it and reopened windows and now optimizing the catalog. Then going to exit lightroom and make sure it opens with my default catalog. Thanks for your fast response.
Everything seems to be working right now. Guess it is just one of those computer things. LOL
You have found one of the LR Automatically created backup catalog files. LR does Auto backups once a week by default.
As to why your original catalog files went missing I don't know. The Catalog file extension is .lrcat. The .lrdata entry is a Folder that holds the Previews of your images. It has the same name as the catalog file with Previews included and then the DOT (.) and LRDATA extension (But it is a Folder).
You should copy that expanded catalog file to the original folder the catalog file was in. That is usually C:\Users\YourUserName\Documents\Pictures and then could be in another subfolder called Lightroom then Lightroom Catalog (Or something like that). Unless you moved it from the Default location.
There are 2 or 3 reasons the catalog file may have vanished.
1) The drive in your system is faulty.
2) You accidentally deleted it when doing some drive cleaning (and or simply by Accident).
3) Some one else that uses that computer deleted it.
I personally move the LR Catalog file out of the My Documents (Documents) folder to some other drive in my system (My LR 6/CC catalog is on my D drive in a folder called Lightroom 6 Catalog and is named Lightroom 6 Catalog. My LR 5 catalog is also on the D drive and called Lightroom 5 Catalog and in a folder called Lightroom 5 Catalog.
The backups for both of the catalogs is on my E drive, in you guests it, in folders called Lightroom 6 or 5 Catalog Backups.