It depends on the size. If it was a large bonfire then you will need to spike and re-seed it. Neighbouring grass will move back in in time, but it would take a fair while to cover more than a few inches effectively.
Following a burn from a barbeque, we spiked and reseeded the fairly small area, ittiook a couple of years to look quite right but is now properly green and thriving. As the previous person said. so much dpeneds on the size of the fire and how deeply it has gone itnto the ground and root systems. You may do better to remove the area and returf or reseed it.