The first step would be to perform a search for SC4 save data on your computer, to see if it's somehow gotten moved somewhere unexpected. Open My Computer (Windows Explorer) and in the top right is a search box, type *.SC4 into that box and hit enter. This should now search your entire machine looking for any files with the .SC4 extension, i.e. SimCity 4 save files. Provided you are searching the entire machine, that should find any such files that are still on it.