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.

The solution for a dead looking lawn in the Autumn is to overseed with English type grass seed.

The solution for a dead looking lawn in and around April time is to plant Warm Season grass plugs.