You've either had new turf laid in the last year or so, or you've got some rotting wood under the turf somewhere are the most likely explanations. Mushrooms and toadstools are just the fruiting bodies of mycelium present in the turf, which you don't notice. Twist them off and bin them to prevent pore spread. They'll pass eventually, but may recur next year at the same time. There is no treatment for these.

Another reason for mushroom growth are dog and or cat feces that is not removed promptly from ones lawn. Also, they grow when there are excessive wet weather conditions, prolonged periods of high humidity or overwatering of your lawn. Other remedies include lawn aeration and using a nitrogen fertilizer, both accelerate the breakdown of organic waste which the fungi feed on and of course less watering of the lawn. As mentioned above there is no cure just palliation of this situation. Fungi are essential to the garden in that they break down organic detritus and release a valuable nutrients into the soil. They are only an esthetic problem or a problem if you are concerned that children or pets may possibly eat poisonous ones.