I can't. On the other folders, I can select Get Info and all that, but on the deleted folder, it only shows Sidebar Preferences
That suggests you deleted the actual Folder represented by that Sidebar item, rather than a duplicate of the Folder.
It's also possible that the Sidebar item represented an Alias, rather than an actual Folder. If you delete an Alias, you lose your link to the Folder, but the actual Folder remains (often, deeply buried). I don't know how thoroughly you used Search... if not, give it a try. If you know the name of one or more Files inside the Folder, search on the file name(s) - an Alias can be given a different name than the item to which it links.
I don't recall the specifics of how Sidebar items were handled by Snow Leopard (been a long time since I've used Snow Kitty), or whether that's been changed at all in subsequent versions of OS X. In El Capitan, if you place an item in the Sidebar, the Sidebar item will disappear if you delete the object it represents. Perhaps in Snow Leopard you have to remove the Sidebar item separately.
Bear in mind that the Sidebar is effectively a shortcut menu - you can't create a New Folder in the Sidebar, you can only add items to the Sidebar if they already exist elsewhere. So, it's possible that what you thought was a copy of the Folder was the actual Folder. It's also possible that you reflexively emptied the Trash after moving the Folder to Trash - I know more than a few people who do that. If you thought you were deleting a copy rather than the original, you might not have thought twice about clicking Empty Trash.
