I forgot to add a picture

Is the dead area totally random in shape, or is it more or less circular? Have you ever tried sticking a 6 inch screwdriver in the soil in the areas of brown grass? If it meets resistence after a few inches it might be a rocky ledge or an area of rocky fill that was applied to the lawn at some point in the past. This can often cause the grass to dry out much faster than the rest of your yard.

Tiemco - It is a semi-circle pattern and it's been the same shape and place for the last 2 years. The lawn looked wonderfully green and lush for the past 20 years, until last year, so I doubt there is rocky soil.

OK, looks like you have a fairy ring. Fairy rings can form areas of dead grass, especially in hot dry summers.

Where do you live?
How have you been watering?
Do you rake the area?
What kind of grass do you have?