Re: The Sims 4 won't open, says i need to go to default apps?

@fairyprincessly  Please uninstall the EA App with Revo Uninstaller (the free version is fine) and reinstall it.  And run both Steam and the EA App as an admin: right-click each desktop shortcut and select "Run as administrator."

If this doesn't help, let me know whether the App is loading properly at all.  A number of players have reported today that the App isn't working, and that may be the underlying issue here as well.