Witches were working, now can't use magic

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.

Sounds like the same thing that happened to me.  Some parts of the game still acknowledge one of my sims as a witch, but she was unable to cast any spells.  The other witch in my household had no issues.  I suspect the game state got corrupted, as this is kind of a standard for the Sims franchise.

Anyhow, I found a way to fix it without having to use the cheats console that was effective.

- Use a Potent Cure elixir on the afflicted sim to return them to human

- Then use a Bottled Witches' Brew on the sim to restore them back to a witch

The good news is that the sim will still be at the witch level prior to corruption, so the only thing lost is a few elixirs

Have you removed all your mods to see if that is the cause of the issue? Try and remove all mods, make your sim a witch again, play for a while, save and quit. Load back up and see if she is still a witch. If she is then it is a mod issue and you need to find which one is causing this. If the issue is still happening then I am not sure. 

As a public service announcement, going into Create a Sim to add or remove an occult state from an already-existing sim can break the sim's occult assignment.  The only safe ways to do this are with elixirs or with a mod like NRaas MasterController.  Either one can fix a sim with an occult state that's already broken.

Since this thread is quite old now, I'll lock it here.