Where did my game go?

Just go return it to Best Buy. Tell them the code wouldn't work and I suggest buying from Origin. But grab an other one off the shelf at Best Buy if you'd rather.

I'm trying to register my game but it says the serial code is invalid/already used

Depending on the situation the solution to this may vary.

You bought the game on Origin
If you bought the game on Origin, it's not necessary to register the code anywhere. Download the Origin Client here: http://origin.com/download, login with the same email you bought the game with and find the game in the My Games tab within the Origin Client to download. You don't need the serial code.

You bought the game on DVD and the box was sealed
Make sure you typed the code right. The following letters look very similar and are easily mistaken for each other

• 0, O and D
• 8 and B
• Z and 2

If it still says it's invalid, try taking it back to the store you bought it from and exchange it. If they are not willing to do that, contact an EA Game Advisor with proof of purchase ready.

You bought the game on DVD and the box was not sealed/it's a used game
Used games cannot be re-registered. Take it back to where you bought it and get a refund.

You bought a key from a key seller
Contact the key seller for a replacement. If you bought the game from a unauthorized seller, it's possible that EA blocked the key if they were bought fraudulently by the seller. You're better off to only buy from authorized sellers.

You have already registered the game yourself and are trying to reinstall the game
The game does not need to be registered again. Login in the Origin Client with the Email/PW you first registered the game on and download and install the game from there. The game only needs to be registered once and you won't need the code again after your initial registration. This is true for both the DVD and the download version. You can skip the activation step on reinstalling.

Source: http://answers.ea.com/t5/The-Sims-4/FAQ-Serial-Codes-Game-Activation-and-Download-Issues/td-p/4228232 

With the issues reaching EA, try a different browser like Internet Explorer, Chrome, Safari, Opera or Firefox.

Your code attaches itself to your account, so if it's the right account EA should be able to find it. 

It needs to be the account you used to register the game with.

Give these a try:

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 exemplary 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

Could you try the call back option?