You can try to restore an old database backup. But if you waited already several days, the graphics files might be gone already.
https://support.plex.tv/articles/202485658-restore-a-database-backed-up-via-scheduled-tasks/
Never, ever fix such issues in the Plex server.
Always update embedded meta tags in your music files, improve file names and folder organization and add cover art as external files.
https://support.plex.tv/articles/categories/your-media/naming-and-organizing-music-media/
You can try to use this: PSA: Script to back up and restore Plex app data on a Windows system
