What type of grass was your lawn? And what spray did you use? Could be that the specific variety of grass was affected by the chemicals in your spray.
The best thing I can prescribe right now is patience - and water. You are going to water the brown grass enough to flush out any remaining chemical residue. Resist the temptation to fertilize your lawn. Fertilizer at this point will only stimulate and stress the grass out more than it already is.