I think I did something horribly wrong... ...help?

I'm a noob when it comes to this and like every page mentions that in big red letters I have to read the steps carefully etc. I did that. Fortunately installing the Sims 4, Get To Work and Get Together no problems occured. I've enjoyed over more than 100 hours of play time.

Now I've installed City Living following the steps very closely again, but there wasn't much to do. I've copied and replaced the files in the correct folder. I didn't realize that I still had the game booted up in the menu. I wasn't able to close it properly so I shut down my computer. 

Now I was starting up the game and everything is gone. The game says it has to ''update'', won't allow mods etc. Anybody have an idea where I could have made a mistake?

//EDIT

Some additional information (I'm Dutch so I'm roughly translating this) the first pop-up says ''The game can't be loaded, Error code 3''

Welcome to the Forum @NeoBowser and sorry to hear about that,

Can you go under Documents\Electronic Arts\ and change the folder name of "The Sims 4" into something else? Then start up and see what happens in the game.