I suspect there is little you can do at this point until your grass grows again, and in this hot weather, without any rain, that is unlikely to happen. If you're not in a hosepipe ban area, I would water it regularly until it's a decent length again, and raise the cutter height on the blades.
I don't think putting any weedkiller on it in this weather is desirable as until the grass is growing regularly again you will find yourself looking at a very ugly patch of dead lawn. If you can't face hand weeding in the meantime, I suspect you'll just have to be patient until the grass grows back.