I would set them up in a shady area of the yard with some pans so they can start swimming in to stay cooler, but it's been a few years since I brooder ducks, perhaps @chickens really has some better advice.
I'm glad it worked. My turkeys do the same thing when the light goes out, they pile and complain, it's their mom. I slowly wean mine off it, leaving it off at times, and turning it on later each evening.
Have you been using a heat lamp with them full time for the past 2 weeks? Sometimes when the light goes out they get stressed. You could try shining a regular light on them and see if it helps. They end up treating the light like it's their mom sometimes.