The Lightroom catalog you want to work with is probably on an external hard drive. The computer you are using doesn’t automatically know about the catalog on the external drive. Double clicking the Lightroom application icon launches a brand new,…

Quit out of Lightroom.Browse the computer to find the folder that contains your photo library. To move the entire photo library, choose the highest level folder that contains all of your photos.Copy this folder to a new internal or external…

Lightroom did not loose your photos, it just lost track of them. You either moved them, renamed them, or renamed the folder they live in.