Pio, RB always keeps a backup each time you close RB, If you're on a mac (not sure the path for pc, you can find it i'm sure) under the username/library/pioneer/rekordbox folder there is a file called datafile.backup.edb
Did you change any folder structures or anything? it's very weird that everything would just go missing for no reason, Not that I wouldn't be the first to blame RB for the screw up also, LOL