When I opened LR 5 I received a message that LR had detected a duplicate file and did I want to delete it. I clicked OK and LR opened but without my current catalog and its contents. I tried Open Recent catalogs, Open catalogs (only the current empty catalog). I then looking for current Backups and there was only one, which was done yesterday, but it was the same one that LR opened with.
I have over 40k photos that I would have to re-import into a new catalog but the biggest job would be Keywording all those photos.
I now have another message stating that the LR catalog could not be found and did I either want to choose a different catalog of use the default catalog.