You will need the old c:\ hard drive (or a backup). If it is not available or crashed hard and cannot be accessed, then the data is gone.
The Plex database, metadata, thumbnails, etc are stored in the Plex Media Server data directory, in %LOCALAPPDATA%\Plex Media Server.
You would need to copy the data from the old drive to the new drive.
See the “Move an Install…” document below for full details.
FYI, it is possible to move the Plex data directory to another location. See the How To document below.
https://support.plex.tv/articles/202915258-where-is-the-plex-media-server-data-directory-located/
https://support.plex.tv/articles/201370363-move-an-install-to-another-system/
