I recently made a Sim a zombie, and when I exited the game was saved. I don't remember saving at all, just exiting to avoid from her becoming a zombie. Is there any way permanently fix this?

Kill her off again, and resurrect her with more money?

The Sim Transformer is one option.


ETA:
@Aegagropilon
I don't think that works. AFAIK, zombies are one "creature" for which there is no in-game cure. Needs a mod of some sort.

The Sim Transformer is the only option; I just tested out my suggestion.

Once a zombie is dead, they can only be resurrected as a zombie. (Which makes sense both from a programming standpoint -- since the character file has been altered -- and from a storytelling standpoint: the Grim Reaper is a Taurus after all, and Taurus sims aren't known for their extreme niceness.)

Well, it's not the only option. I'm just having trouble remembering details about other options, and so can't offer them right now. Pretty sure there's a cure potion or two floating around somewhere, as well as perhaps something like the Insiminator?

I haven't tried either one of these, but for what it's worth, these cures were found at this site:

http://www.modthesims.info/download.php?t=209590
http://www.modthesims.info/download.php?t=283201

P.S. Google is your friend