There are usually two common Summer problems that cause this symptom:
1) Fungus Damage
2) Insect Damage (mostly chinch bug and sod webworms)
The correct diagnosis is key.
Time is not on your side. If I had a dollar for each person that came in and starts by saying "I was out of town for a few days and I came back to a huge brown spot in my lawn"
If you mis-diagnose and use an insecticide when you need a fungicide...guess what? You just lost 100 sq ft of lawn. These problems progress very fast...one of the reasons it's difficult for a monthly lawn spray service to be very effective.
If this sounds familiar stop in for a diagnosis. Bring us a 6" x 6" square of turf that is at the edge of the damaged area (it should have grass in transition...going from green to brown). Also bring the soil with it....that's where the insects hide!