Well , my game's still working fine , only that it's not the same anymore. A few days ago my sister messed my saved game up , using" Trigger age transition " cheat to age up random NPCs and my sim's babies. Now the town's full of adult and elder sims. Is there a way to reset the whole town back to how it was , like when you first start a new game? I don't mind if my sims lose relationships with other NPCs and such , as long as they stay the same. Thank you

If game isn's saved again after she "changed" things, you will have a folder with the same name as your save but with the extension .backup in your saves folder, delete or rename the current save then remove the .backup and you will have game back as it was before the last save.
If it have been saved again after the "change" only way to get things back is to start a new game/town, if you created the family your self it is saved in the game bin, they are however as when you created them.

The NRaas Porter (http://nraas.wikispaces.com/Porter) might help you with this. i.e. You can save all the Sims you want to keep, plus their family-groups and relationships in one big Uber Household. Then you can start a new game and automagically unpack them around town.

Not exactly ideal, but it's the only think I can think of.

If you want to retrieve the town as it is when you start a new game save your household (edit town, click on your lot. Save with the house to keep all your belongings), exit to main menu then start a new game with your town, your household will be in the bin. Place them in the hood and you're set. Theorically you keep the relation with the NPCs since they're the same ones, only younger.

Thanks everyone , I moved the whole family to a new game and everything's fine now