While I'm usually in agreement with Thunderbird+, in this case I'd advise that here in the U.S. all newer weed killers are required to be neutralized in the soil after a very short time... in some cases 48 hours but in any case no longer than 7 to 10 days. Depending on the active ingredients of the weed killer you used (see the label) I would wait about a week or so, and then scarify the top soil down to about 1 to 2 inches, rake off any dead grass, spread granular fertilizer at about 3 pounds per 1,000 square feet, seed with a good quality grass seed and cover with a top dressing of a manure-compost mix to about one half inch. Keep the entire area damp and when the grass sprouts use at least an inch of water per week. Best of luck!