If the file is good and truly deleted, you can restore it if you are using a system like TimeMachine or another backup service. If you're not, you should be.
But! If the file isn't actually deleted, be sure you're looking in the right place for it:
Win 7/8: C:\Users\yourusername\AppData\Roaming\Pioneer\rekordbox
OS X: /Users/yourusername/Library/Pioneer/rekordbox
You could also check to see if you have a backup file there if the main database is damaged / missing:
Rename datafile.edb to datafile.edb.old (if it exists).
Rename datafile.backup.edb to datafile.edb (removing .backup).