I wouldn't think the Fixated moodlet would be strong enough to cause that effect. Did your Sim have any other negative moodlets contributing to her mood? If you need more in depth troubleshooting on this issue, please join us on Discord and I or one of my friends can take a look.
Nothing else usually! I’ll try and cheat them up to 100% everything when it comes to food, fun, etc, and the moodlet still makes it so they can’t do anything. Usually I have my sim just watch TV or wind down with classical music for a couple hours but then they get sad from “lost idea” moodlet which I don’t know how to have an alternate outcome.