The heat of summer can cause stress to your lawn. If we have day after day of very little rain and we have temperatures in the 80's and 90's, your lawn will suffer greatly without enough water.
You can bring it back to health by increasing your watering schedule to every day until it shows signs of returning to a happy lawn. Once it starts to green up again, you can go back to your regular summer schedule of 30 - 40 minutes per zone every other day in the morning hours.