Hi, with a bit of TLC, you grass will grow back again... what I would recommend is that you rake out the "brown" grass - get back to soil level, rake the top few centermetres of soil to loosen up, water the area and sow some grass seed, keep the area moist & you will soon have new grass growing... if the soil ain't too good, then you can buy a bag (depending on the area affected, may need more than one bag) of top soil from a place like woodies/B&Q and rake into the area...
What type of plant caused this?