A lawn can appear dead at different times of the year.
For example if you have a warm season lawn, it may well go completely brown (looking dead) in the Autumn or Winter (see website for details).
If you have an English type (Cool season) lawn it may well go completely dead when the temperatures rise around April time.

But an added complication is where you have more than one type of grass.
For example if you have areas of your lawn that contain Bermuda grass, then these areas will appear to go dead in the winter.

The solution is to remove the unwanted grass and replant with more of the type of grass that represents the majority of your lawn.