Was your backup on the external drive created with the Organizer? If so, simply restore the backup.
Did you also replace the system drive on your machine? If you still have the original system drive, did you do a fresh install of Windows or did you save the data files with a Windows renewal? Where were the original photo files stored?
It is possible to relink the thumbnails with the D drive photos, but it can be a tedious task, especially if you have a large catalog. Without more information about your old and new setup, it is difficult to give you some meaningful advice.