@King Kong > What version of OSX are you using?
It could be that your database is corrupted or damaged and you need to try loading the backup database. Ensure rekordbox is closed and find your database in the following location:
XP: C:\Documents and Settings\yourusername\Application Data\Pioneer\rekordbox
Win 7: C:\Users\yourusername\AppData\Roaming\Pioneer\rekordbox
Rename datafile.edb to datafile.edb.old
Rename datafile.backup.edb to datafile.edb (removing .backup)
Restart rekordbox and see if it opens. If it does, the main database was corrupted and the backup was working okay.
If it does not open, go back and rename ALL of the datafile files in that folder to .old and restart Rekordbox once more. If it does load, both the original and backup database were corrupted and you will have to re-import your songs once more.