BriarMuse Please check your antivirus for any recent actions involving Sims 3 or the EA App. If you find something, set the necessary exceptions/exclusions. Even if you only use Windows Defender, Controlled Folder Access could be interfering.
If that's not the issue, try clearing Windows temp files:
- Click Windows key-R, and a run box will appear in the lower-left corner of the screen.
- In the new window, type or copy & paste "%temp%" (without the quotes), and enter.
- Now your Windows Explorer should have brought you to the C:\Users\[your user name]\AppData\Local\Temp folder. Delete all files and folders inside.
- If some files or folders can't be deleted, restart your computer (a real restart, not sleep/wake) and try again.
Additionally, before trying again to open the EA App, clean-uninstall it with Revo Uninstaller (the free version is fine). Use the Intensive scanning method when asked. Restart your computer and download a fresh copy of the App installer from here:
https://www.ea.com/ea-app#downloads
If that doesn't help, you could try blocking Sims 3 from the internet—maybe that would leave the rest of your computer alone. The simpler way, if it works, is to put the EA App in offline mode before you launch the game, but I don't know whether that cuts off everything. If it doesn't help, you could block Sims 3-related data through the Windows built-in firewall.