I found that if you rome the modlet that cased the sim to die i.e. death by fire the ghost comes back to life, but you need a cheat which you can get from Mod The Sims, enable advanced debug/cheat interactions, once the script has been placed in your mod folder, start your game and then select your ghost and shift clicking on sims, use the more choices tab at top of menu and select remove modlet, scroll down until you find the death modlet and just remove it and bingo your ghost is a live, hope this helps.
