layniiiiii If you create new sims in one of the saves, and place them in the world, can you find your old sims either in their houses or in Manage Households? If so, can you play them?
If you can play an older household, let the game clock run for a bit and use "save as" to rename the save. Quit, restart your computer, try to load the save again, and let me know what happens.
If you can't play an older household, but you see them somewhere in-game, try saving them to the library and see whether they place in a new save. Quit and reload the game in between steps.