How long ago did you apply the weed killer? It stays viable in the ground for about 6 months. Afterwards, you'll be safe cultivating new grass.

depends on the type of weedkiller, some become inert when in contact with the soil. remove the dead area, rake fresh soil into the spot and resow seed in march/april.

And next time, use a Lawn weedkiller such as Verdone 2...