I just opened LR 6.3 to what looks like a completely empty catalog. Zero photos in catalog, no folders/collections/etc. I have verified that I'm using my most recent .lrcat file (dated Dec 1, 2015) and tried opening all other catalog files that I could find on both internal and external drives (all of which were from older LR versions). The .lrcat file that I'm using is 1.41MB in size so there's obviously something there. Last things that I remember doing are:
Any ideas what might be happening? My backup location (another computer on the network) has been down for a few weeks and don't have a current backup. I hope this is not a painful backup lesson for me and that I haven't lost all of my edits from the last couple of weeks. Thanks for any help!
1.41MB is tiny and would indicate an empty catalog to me. Files and presets are external to the catalog. I think Lightroom may have generated a new catalog for you. As to the location of the previous catalog, it's usually in Pictures/Lightroom or My Pictures/Lightroom.
From the menu, check your "recent" catalogs to see if LR opened a different catalog than you expected.
This can happen often during an upgrade.