how do I unhide a photo album

You may need to click on "View>Show Hidden Album" like this,

to make the album appear appear in the sidebar above the "Recently Deleted" view.

You can't hide an album, only photos in a Photos library. Do you have an album in which the photos that were in there are hidden? If that's the case and you'r running Ventura click on the Hidden album in the sidebar and enter your admin password for the Mac. Once in the Hidden album select the photos you want to unhide and use the Images ➙ Unhide X Photos menu option.