nothing wrong, it is not only your lawn , it is every body's lawn turn yelllow. Grass would go dormant in hot weather, it turns yellow. It is not dead. It would turn green when temperature goes down and few good rain. Usually grass go dormant in July in ottawa if you don't water them. But this year, May is hot. So they go dormant. Don't worry, they are fine. Deep water them. They would back to green.