My Lightroom 4 wouldn't open. A computer tech worked on my computer to clean it all up, etc. however Lightroom still wouldn't open. He opened a new catalog and that will work, however I can't find my old one with all my photographs. When I click on open catalogs, it goes to documents and I can't find any catalogs there.
What happens if you try File->Open Recent? What happens if you have your operating system search all of your hard disks for files whose name ends with .LRCAT?