Guest, need a photograph - toadstools often grow in lawns, and then just disappear, and do not harm the grass at all, but it sounds as if you have more going on, because you mention a dead area of grass. If this dead area of grass forms a ring , with toadstools at the outer edge, green grass in the centre, on the other side of the dead area, you have a serious problem which is hard to cure. But without more detail or a photograph, hard to say. There is no cure for fungal growths in lawns, although iron sulphate sometimes helps - the fungus growth is usually caused by buried woody debris under the lawn.

Regular applications of composted cow manure, or compost tea, will usually reduce lawn toadstool problems by giving the toadstool fungus some competition from other organisms.