If you did not use grub control this year or it was applied at the wrong time you may find your lawn looking like this. A simple way to see if you may have grub damage is to do the tug test. Try and grab the dead grass with your fingers and lift. If the dead grass and roots do not move it is most likely another problem. If the dead grass lifts right up you will see the grubs feeding on your roots below.
You will need to treat the area with a grub killer that is labeled to Kill Grubs on contact. You do not want to use Grub X or a Merit product, it is too late for this. Unfortunately the damage has already been done and likely to get worse if a control is not applied. Re-Seeding will likely be necessary for damaged areas.