Sims 4 is gone from the "my games" tab on origin

Are you in the right account?

The game no longer shows up in Origin

Try the following things: 

• Restart Origin.
• Log out and back in and use the email address instead of the user name. Make sure to use the one you used to register the game in the first place.
• Reinstall Origin.
• Restart your router/modem.
• Make sure the game isn't hidden by accident. To check, put the filter to Hidden Games. If it's there, right-click on the game to unhide ((click on the spoiler below to view an examplary screenshot))
  
  
  Spoiler

   
• Get in touch with an EA Game Advisor.
  
  Source: http://answers.ea.com/t5/The-Sims-4/FAQ-Serial-Codes-Game-Activation-and-Download-Issues/td-p/4228232