-: ATTN - if your game won't start, READ HERE :-

Move your Sims 3 folder to your Desktop (Documents > Electronic Arts > Sims 3) and start the game back up. You won't have your saved games or CC (Yet) and it clears whatever happens to prevent the game from running. I use Time Machine and so I go back to an earlier day (when the game was working fine) and restore my Sims 3 folder from there, but I take out my most recent saves and add them back in. That seems to clear whatever was preventing the game from starting up.