Are you trying to play an old game? By that I mean, are you loading a save file that you have been playing for a lot of Sim weeks? If so, it may be the game file is corrupt or too big for your machine. If you start a new game, can you play it successfully?
If it's the game file itself, Twallan (also known as Nraas Industries) provides a couple of mods that can help you solve that, specifically Overwatch and ErrorTrap. You can learn more about these mods in the Game Modifications section of this board. There are a few other things you can do to help clean up your game, like deleting memories and clearing out your Sims' inventories.
There are also some pinned threads here with some great information that may be helpful:
http://www.carls-sims-3-guide.com/forum/index.php/topic,463.0.html and
http://www.carls-sims-3-guide.com/forum/index.php/topic,8211.0.html are a couple of them.
Additional information that might help the forum to help you would include what version of the base game and any expansion packs you have installed. Also - does the freeze occur when your Sims are trying to perform a specific command? If so, it could be related to that specific action.
