What has happened, before the Shared Albums disappeared?
If you turned off the Shared albums in the Settings and then turn them on again, it may take a few days, until the albums reappear. and only the photos that are still in iCloud will reappear. Items that have been deleted from the Shared albums, are gone for good.
Are the Shared albums currently turned on in the Settings > Your Name > iCloud > Photos?
Shared albums, that have been deleted from iCloud, can only be recovered from your own backups you made on a computer.