Both cool-season and warm-season grasses go dormant as a coping mechanism. Warm-season grass, such as Bermuda grass, go dormant during cooler weather. Cool-season grasses, such as Kentucky bluegrass, go dormant during both hot, dry summers and cold, snowy winters. When grass goes dormant, it turns brown and looks like it is dead. The difference is that dormant grass's crown, the part from which the grass grows, is still alive.
Using a soil thermometer, take the soil temperature in several locations of your lawn. If you have warm-season grass, the soil temperature must be between 55 and 65 degrees Fahrenheit for the grass to begin growing. Cool-season grasses require a soil temperature between 45 and 55 F for growth to begin. If the soil temperature is below the required minimum, your grass is dormant. If the temperature is at or above the appropriate temperature, but the grass is not growing, it is probably dead.
Pay attention to how much water your lawn is getting. A lawn needs approximately 1 1/2 inches of water each week. If the weather has been hot and dry, and your grass is turning brown, chances are good it is going dormant. If your grass has received enough rain and the weather has been temperate, your grass is dying because of something else, such as a lack of soil nutrients.
If you suspect your lawn is going dormant because of heat and lack of water, walk along your lawn and look closely at the grass blades. If green is still visible in many blades, your lawn is going dormant and should perk up once conditions improve. If there is little to no green in your lawn, expect large portions of your lawn to die, even if conditions improve.