Does the backup drive have the same drive letter like the old (failed) drive?
It should.
You may have to restore an old database backup from before the drive outage.
https://support.plex.tv/articles/202485658-restore-a-database-backed-up-via-scheduled-tasks/
To prevent this from happening again, disable
Settings - Server - Library -‘Empty trash automatically after every scan’
You must never activate it if your media storage is not inside the Plex server device.
