What's the easiest, cheapest way to bring a patchy lawn back to life?

the easiest and cheapest way is to do what i did now i have a beautiful green lawn you need to go to lowes or home depot and by grass seeds called quick start you then go home and just sprinkle the seeds in the patchy areas and water them every day in the after noon in 3 to 5 days you wil have grass the bag says temporary but i done my lawn 3 yrs ago and the green grass is still there its not really temp grass the bag just says that

It depends on why it's patchy. If you've watered and fertlized and you dont see any response it could be bugs that like to eat your roots. you'll need to find a fertilizer that is mixed with a pesticide. Or it could be disease, there are fertilizers for that too. You may have to try a few things before you see any results.

You need to find out why it's patchy. If you have a fungus or mold problem nothing will stay alive. Call a professional. They sometimes will come for free.

Aerate your lawn very thoroughly. Go over it with the aerator a few times. -Then after a few reseed the whole lawn with a clumping type of grass seed.

Grass seeds covered with hay. Water properly thereafter.