We would have to know more about your situation to give you a precise answer. Did you fertilize in the spring? Plant in the spring? Age of the lawn, fertilization, liming, soil testing? Have sun or shade? Type of soil--sandy, clay, level, slope? What is the pattern of browning? When did it turn brown, how fast, etc.? Or anything else that could be pertinent.
Meanwhile, read through this basic lawn success fact sheet: http://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG102%20La...
I have fertilized for the past three years with Scott's 4 step program. In early June, I applied the #3 bag to my lawn ( 5,000 sq.ft.). Within 3 weeks, brown patches appeared, with dead grass following up.
The yard is 24 years old, and all spring I was complimented by neighbors for how great it looked. It was definitely the best in all those years. Between rain and watering every 3 days for at least 30 minutes per section, I don't believe it dried out. The areas most affected are in full sun all day. Soil is clay.
Many of my neighbors have had similar problems.
As soon as I can figure out how to get photos off my phone to the computer, I'll attach a few.
It would help if you could send us a few photos. Some types of grasses (annual rye) die back in the summer, and some types of grasses go dormant at this time of year, so the dieback might have to do with the type of grass you have. There is also a fungal disease (brown patch) that is active at this time of year when we have warm weather and high humidity. Take a look at the following page, which describes the symptoms of brown patch and what to look for. https://extension.umd.edu/hgic/brown-patch-lawns