@Maddy1881 Seasons requires a fair amount of resources from the computer that's running it. You could be out of RAM, or your graphics card might be underpowered for the EP. Or your game folder could be corrupted. Or a couple of other things. So it's best to narrow it down.
First, drag your entire TS3 folder out of Documents and onto your desktop. Then launch the game again. A clean folder will spawn with no content. Try launching the game and playing for a bit in this folder. If you can spend a bit of time in CAS, move in a family, and play with it for maybe half an hour, try to save, then quit to desktop, then relaunch the game. Can you load this save again? If so, you can move your content from the old game folder on your desktop into the new one, a little bit at a time. There may be something in this folder that has become corrupt, so if anything causes the game to be unplayable again, it's probably corrupt and should be trashed.
If you can't play properly even in a clean folder, then it's more likely (but not certain) that your computer is the problem. So please either post your computer's specs (mac—see the first link for details) or run a dxdiag (PC, second link) and attach the file to a post here using the paper clip icon.
https://answers.ea.com/t5/The-Sims-Mac/READ-FIRST-Sims-3-Mac-Help-Guides-and-Troubleshooting/m-p/4756624#M4
https://answers.ea.com/t5/The-Sims-3/HOW-TO-DxDiag-PC-Only/m-p/1049584/thread-id/47181