Many things can cause a lawn to die. Animal urine can destroy it. Birds can damage it by pecking for worms in your yard. Insects can cause grass to die in places, too much sun can scorch it as well, and if you don't get enough rainfall to keep the lawn supplied with moisture, it can die. Lawns are like humans: they need sunlight, water and nutrients and protection from pests.
Dying lawns can be due to a lack of nutrients in your soil. Your lawn and plants cannot grow without nitrogen, phosphorous, potassium, magnesium, and other nutrients such as iron and zinc. In addition, grass needs chlorine to maintain a nice green color. Adding a fertilizer that combines these components can get your dying lawn back to life in a matter of a week or two, but you need to make sure that this is the problem causing your lawn to die first, and you do that with a soil analysis test.
Sometimes adding fertilizer isn't enough to save a dying lawn because the soil is too compacted to allow the fertilizer to penetrate. In this case you will need to use an aeration machine on your lawn to open up the soil. This tool is basically run over the lawn, where it pushes holes down into the soil, enabling your fertilizer to penetrate the ground and provide nourishment for your grass' roots.
If fertilizing and aeration of your lawn fails to revive your dying yard, you can reseed with new grass. This is the time to evaluate the grass originally sown, to see if another grass might be better suited for your geographical location or conditions, and to sow it. Some grasses are hardier than others and can withstand drier spells better. Choose a grass for your particular planting zone conditions and sun exposure as well as expected rainfall.
As a last resort for a dying yard, you can place sod down onto the lawn. You will need to remove all the existing grass first, plowing up the yard and then leveling it again for the sod to be sown. Laying sod can give you an instant new lawn that looks great, but it is an expensive enterprise and not one to enter into if you can save your dying lawn instead.