Do you mean the cat is in your household but you can't select it? If so, try going into Edit Town, selecting a random family, do "Shift Ctrl C" and put "Testingcheatsenabled true" Then, go find the cat. Once finding the cat, shift-click on it and click "Add to Active Household" or something like that. Then, go back to your original family then do the same thing and add it back. That should fix it. If you mean it came back and isn't in your family, simply do again "Shift, Ctrl, C" then "Testingcheatsenabled true" again and add it back to your family. If you don't want to use cheats, then you'll probably have to wait till it becomes an elder and well, dies. Then you can get the task for the science lab "Oh My Ghost!", pick up the grave and get a ghost cat. Hope this helped!