Most fish do not die when lakes freeze over because fish are cold blooded and can handle the cold.
Although fish do go into less activity during the cold weather and when the lakes are frozen so fish require less food to eat when the weather is freezing and the lakes are frozen.
It is possible for some fish to die when lakes freeze over but it's not because of the cold but instead the reason some fish die as a result of the lake freezing over it's because of the lack of oxygen getting into the water from the frozen lake being sealed from the outside oxygen.
When the fish don't get that oxygen then they can die.
But it's rare that fish do die as a result of lack of oxygen when lakes freeze but it can happen.