You can edit your sim to be a witch by holding: Ctrl ALT and C together and then entering in the bar "Testingcheatsenabled true" and if you then hold ALT and click on your sim with the problem, you are able to edit sim and hopefully change them back to a witch.
Hopefully this works, sorry if it doesn't ! 🙂
Also this could be effected by if your sims magic meter has been depleted; is empty and so she will not be able to cast any magic. Her bar is filled back up when a full moon is back.
