All your libraries, all your metadata were stored on your old hard drive.
The basis is this procedure: https://support.plex.tv/articles/201539237-backing-up-plex-media-server-data/
For Windows, you’d not only need to copy the old Plex data folder, but also the contents of the Windows registry branch that pertains to Plex.
Use this for details: Can't transfer play history to new server
