I have had this problem happen once before. Virtually all my catalogs and photos in Lightroom have disappeared. The Lightroom FAQ has a suggested cure, but that is not proving helpful. I used the phone support several months ago to solve this problem, but that's not an option this evening. Can someone explain/lead me through restoring all my missing catalogs and 74,000 photos.
Check your computer's "recycle bin" and restore them if necessary.
Use Explorer or Finder to do a search for *.LrCat files on all your hard drives.
What did Customer Service do to solve the problem last time?