I recently went to do some editing in Lightroom and found that my whole collection of pics has disappeared.
You most likely (accidentally?) opened a empty catalog file. Go to File->Open Recent and select each catalog listed there until you find the one with your photos.
I tried doing that and the Catalogue I'm looking for just doesn't seem to be anywhere.
Use your operating system's search feature to search all of your hard disks for files whose name ends with .LRCAT.
Double-click on each one that is found to open it.
Also, you need to be making regular and automated backups of your catalog file and all photos. The catalog file can be backed up via the built in Lightroom function, which can be turned on in the Catalog Settings section. You need to make regular and automated backups of your photos by some third-party software.
Ok. Thanks for that.