Unless you intentionally moved a bunch of additional files and folders to the D drive, there would have been preference and settings files stored and maintained on the C drive (under your user's folder).
And unless you specifically moved the database to the external drive using the advanced page in the preferences, only the music would have been on the external drive; the database file itself would have lived in the same user folder on the C drive as your preferences.
The Pioneer folder you refer to may be what's needed, depending on what's still in there. Can you post a screenshot of the contents and provide additional answers to clarify the above?