May I add, if you do check for chinch bugs (which would be my guess on the problem) do the test on the edge of the damaged area, where the green meets the brown. That's where the bugs will be.
If you do find chinch bugs (black small beetles with a white stripe across their backs) head to a good nursery for the right insecticide. Usually it's not necessary to treat the whole lawn with the stuff, and the less insecticide in the environment the better, of course.
