So I tried running the Sims again and this message appeared:
It looks like you have installed an unofficial game modification that does not match your current version of The Sims 3. We recommend that you either uninstall these game modifications or get updated versions of them – otherwise you may experience crashes or significant bugs.
Is the unupdated CC causing this issue? It's ran perfectly fine while displaying this message before. After selecting
"No, I don't want to quit" ,
The screen went black and the Sims refused to open. As mentioned before, the Sims has (previously) opened while displaying the old CC warning.