In order to find the catalog, you may search your computer using search string "lrcat". This will provide you all the catalogs available on your computer.
Also, you should search your external hard drive if you are using any.
Also, please provide the exact error.
Which Lightroom version you are using?
Which operating system are you using?
Lightroom recently had an error with a catalog and has lost all of my photos.
For clarity, your photos have not been "lost". Regardless of what might have happened to your catalogue, your photos are still in the location you told Lightroom to import them to.
They are not, and have never been, in the catalogue, because the catalogue simply contains information about the images - not the images themselves. They're still sitting on a hard drive somewhere.
How do I get the catalogs that were "lost" to correspond to the new catalog that was made?
Hold down the option or alt key while clicking to open LR. A box should pop up that will list all of the catalogs that are on the computer or hard drive. You can also create a new catalog from this box if you want to do that. It's important to know where you are keeping your photos, your catalogs, and your backups on your hard or external drives. When you load a photo into LR it is simply linked from the place where the photo resides on your hard drive or external drive to the left panel that you see in LR. That left panel in LR where you move and use folders is a replica or mirror image of what is on your hard or external drive. Normally, on a Mac, the catalog and photos are placed in the Photos folder in the finder, not sure about a PC but probably the same. Look in that folder first. You should be sure to back everything up to another drive so that if your hard drive on the computer, assuming that is what you are using, goes down you can obtain what you need from the external drive. You can find some additional information on importing at viathelens.net.