You didn't say where you're from, so I'm just guessing about the type of grass you have and what would cause this. However, this time of year -- when the summer heat really starts to come on and stress the grass -- is when summer patch and brown patch show up. Make sure your grass is well watered. Apply the water in the early morning so that it won't stay wet very long before the sun dries it out. And make sure your mower is set pretty high -- 2-3 inches. Fungicides are available at garden centers (that's your fallback if it really gets out of hand), but my experience is that good management usually is adequate to keep diseases under control. A spreading grass such as bluegrass will usually fill back into the dead spots on its own, so minor outbreaks will "heal" up on their own. Larger dead patches can be reseeded in fall.