I lost my case with the serial key

You can get serial codes on the internet that will work the same, but I wouldn't suggest it.

You can use serial codes as many times as you like (that I know of) , so don't worry, or you could contact EA, but this is much easier 🙂  

Your lucky, I lost my disk but have the serial code ☹️ 

First off, do not use the code listed above!

Do you have access to computer where the game is currently installed?  If so, you can find your code yourself (http://answers.ea.com/t5/The-Sims-3/FAQ-Lost-and-broken-DVDs-and-missing-serial-codes-What-to-do/td-p/14230)

If you don't, but you registered the game with Origin/ the Sims site, you can get help from EA and they can retrieve your password and/or give you a new one.  Here is a link to contact them: https://help.ea.com/contact-us

Also, here's a post on how to navigate the contact page in case it's confusing: http://sims3.crinrict.com/en/2012/10/how-to-contact-ea.html (Just so you know there is no longer an email option, only live chat and phone.)

Best of luck. 

Can I ask why they can't just use any code, it doesn't make any difference unless you want  to retrieve free points.

One because it is illegal to do so and against forum rules to post any kind of serial code like that.  

Two because a serial code cannot be used an infinite amount of times.  There is a limit.  

Three because she won't have full access to the Sims community since that code has most likely been registered before.

Four because it is dangerous to use random codes you find off the Internet.

Thanks for letting me know, I had no idea it was illegal to use codes of the internet and I honestly wouldn't post it here if I had known and was just trying to help in a simple way that I found from searching the internet, in the past I have never lost mine. Also I have removed the code in order to not be against any policies. 🙂 Hope you resolve your issue so you can continue playing.