Sims 3 Seasons - Lost Serial code

Here's a very helpful link from EA.: http://answers.ea.com/t5/The-Sims-3/FAQ-Lost-and-broken-DVDs-and-missing-serial-codes-What-to-do/td-p/14230 You should do this: 

Finding your Registration / Serial Code

If you still have the game on your computer you can follow these steps to find your serial code from your registry (PC only). Credits of to Carl's Sims 3 Guide

You need to be signed in as an Administrator first.  Windows may require your permission to open the
Registry Editor and if so, just click on yes.

1. Press the Windows key on your keyboard and type regedit in the search bar and press Enter. 
2. Click on 'File' at the top left of the window that opens then click on Export.
3. Choose a location and name for the file as this will be your backup registry file.
4. Click Save.
5. This backup is created just as a precaution.

Now to find your serial codes for each specific EP or SP installed on your computer:

1. Expand the HKEY_LOCAL_MACHINE folder.
2. Expand the SOFTWARE folder (Wow6432Node for 64 bit systems)
3. Expand the Electronic Arts folder.
4. Expand the Sims folder
5. Expand the EP or SP you need the code for
6. Click on ergc and in the box to the right under 'Data' you will see the code
7. Write it down somewhere save
8. Repeat above steps for each EP

Do it all at once! The number of the steps doesn't matter. You have to do all of the steps to get your code. I hope this helped you! You're welcome!