That really depends what's wrong with it. There are many problems that could be needing fixing. If the problem is simply turning on the game (i.e. the game will not be recognized), the best option is to gently wipe off the game disc/cartridge of any wipes, smears, etc.
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You don't do that in sims 2 only in the new game - sims medieval.
Restart your computer or reinstall it. it could be a glitch or its a game problem that needs to be reinstalled
Your computer may not meet the sims 2 game requirements.
if you have The Sims 2 and The sims 2 pets, install the sims 2 first and yes you need all four discs and the game case with the code. Then you can install the sims 2 pets and you will also need the manual for the Sims 2 pets or the game case for the code to install the game.
Sims Castaway is a totally separate game.