I don't think you can restart one story, but you can reset the whole game without uninstalling. It will tell you on the first page of the manual thingy.
You have to uninstall it and then Reinstall it. It really stinks but that's what i did once to restart something that I messed up. If anything, Try this and this is the only thing that I know how to...
yes you can reset the story lines. It means completely resetting them, so no going back. Ok back up your data from my documents files (the sims life stories neighbourhoods noo1) then goto where ever...