I believe they'll just go into a state of torpor, but if it's not cold enough, they might starve instead. Can't you bring them inside for a while and fix the pond? Perhaps just buy a liner to put in there for the winter.

I'm trying to hibernate my turtles so I know this is tricky too. Where I live the temps don't get below freezing, but from what I've heard, the turtles will just linger at the bottom.