When you remove photos from Lightroo, you lose the collection information for tthose photos. You may also lose your develop settings for those photos.
If you have a recent backup of your catalog file made before this happened, restore that backup catalog and your collections and edits will be in there. Otherwise, the information is gone.
in the future, never reimport. Move the photos in your operating system and then reconnect in Lightroom via http://www.computer-darkroom.com/lr2_find_folder/find-folder.htm