That must be very frustrating, as it is hard to even play Sims 2 without cheats.
I know this is obvious, but make sure you are pressing them all at the same time, and in the correct order (control, then shift, then c)
The command to take a picture is similar to the cheat command, so make sure that you are pressing all the correct keys.
If you are using a wireless keyboard, try new batteries.
Perhaps one of your keys does not work. This is probably not the case, but you might as well try the same keys on the other side of the keyboard.
The box that shows up when pressing CTRL + SHIFT + C is not very noticeable. If you do not know already, it is a grey box located in the top left corner of the screen. Make sure that the box is up there, because it is very hard to see.
If the above solutions do not work, I would contact EA Games and see if they have any solutions for you.