I moved into a house 3 years ago. My lawn was beautiful, then in the 3rd year it looks terrible with mostly weeds and bare patches.
My Grass Is Dead
Certified GKH Gardening Expert
It is possible that the previous owner (unscrupulously) planted a short life grass, like ryegrass. It grows quickly, looks lovely for about 1-3 years and then dies. It is normally mixed with another kind of grass seed to fill in the lawn while the slower growing, but longer lived grass seed gets established. Unfortunately, for a less than honest home seller, unmixed short life turf seed can give you a cheap, fast, gorgeous lawn while you sell the house and the buyer is left holding the bag when the grass dies off.
The other possibility is that you have a fungus that is killing the lawn. This article outlines the most common kinds of fungus that attck lawns: