Witches were working, now can't use magic

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