You may have corrupted or otherwise incompatible custom content. If you can determine the items, temporarily take out anything you put into your game since the last time things appeared noirmally.
Or you may have a corrupted or otherwise incomplete default replacement skin. Try temporarily taking default skins (if you have any instlled) out of your game.
Or you may have more than one default replacement skin of the same type in the game. Only one default replacement of the same type can be used in-game at any one time.
Or you may have a corrupted objects.package file. See here:
http://www.sims2wiki.info/wiki.php?title=Game_Help:Corrupted_Objects.packageNote: Make a back-up of your entire My Documents--EA Games--The Sims 2 folder before following any of the steps outlined at the link.