Sorry to hear you're having trouble. If we're to help you'll need to provide a lot more detail. What OS? What files? A system backup? Catalog Backup? Etc
Detail the process that has you where you are.
I use OS X 10.9.5 on a Macbook Pro. My entire lightroom library that was in the "all photographs" tab on the left side of the screen. I was looking for a previous group of images on a catalog backup and when I loaded the file it seems to have erased or made my catalog all go blank.
A catalog file only contains references to images imported into. If you open another catalog, it won't have the same references. It may have the same images, but they may not even be in the same place. Go to the FIle menu and from Open Recent, choose the previous catalog you had open.