How did you delete them? From the organizer or from someplace else, like Windows Explorer or the Mac Finder?
Go to Folder's view and find the folder name you had thise photos placed in.
If you find that folder, click on it to view the photos from that folder, which are already present in your catalog.
Sometimes, it so happens, that you import and you get a message for one or two files that these are already present.(these can be the ones which you missed deleting.)
Hence, when you try again, you cannot import them again.
Please check that all options in View >> Media Types are checked.