You can do something about it but it will require some work, patience and cheats 🙂
First, you need to check at what age the Social Worker took your Sim.
If the social worker took your sim as a child, congratulations, you are in luck.
Follow this link to view full article and guide: http://excusemygeek.com/2011/11/07/how-to-get-your-sim-back-from-the-social-worker/
This post was written in response to: http://answers.yahoo.com/question/index?qid=20111019220605AAXhgP5
Image from: http://sims.wikia.com/wiki/Social_worker