A bird hits your window, drops, and lies there limp. Before you assume the worst: a lot of the time, it's not dead, it's just stunned.
What actually finishes it off is the cat that finds it lying there helpless. So buy it time. Set a colander over it, upside down, right where it landed. The holes let it breathe and see out, and nothing can reach it while it comes around.
Two things to think about though: Keep it in the shade, never the sun, because a metal colander in direct light turns into an oven and you'll cook the bird you're trying to save. And don't walk off and forget it. Lift the colander every fifteen minutes, and the second it's alert, let it go.
Don't feed it, don't try to give it water, and don't handle it more than you have to. If it's still down after a couple of hours, or a wing is hanging wrong, it's hurt worse than time can fix. Call a wildlife rehabber.