Only time will tell, but I would water it well, hit it with a Garden Weasel cultivator, a half rate of starter fertilizer and seed now. It may come back from the roots, the seed may take or it could be one big dead spot. However it turns out, overseed it in fall.

 

^+1 I would also mow it shorter to remove the dead stuff. I think it will recover.

 

Probably the heat did it. Generators do the same. Or possibly an oil leak from the truck. How long did it take to go brown? And was it parked in the grass itself while running? Any oil in the driveway?