Hummingbirds can become hypothermic if they sit on a feeder perch and drink several cropfuls of cold sugar water early in the morning. The birds fall to the ground unresponsive if they become extremely hypothermic. If they are not found before they die and warmed up, they just lay on the ground or in one case on the chair until they die. The question is, is there a hummingbird feeder with perches on it near where the hummingbirds were found?
The little birds may have been exposed to insecticides, like Imidacloprid by eating contaminated insects. It doesn't take much Imidacloprid to kill a bird as small as a hummingbird. Imidacloprid was recently shown to cause birth defects on newborn white-tailed deer and also kill the adult females and their fawns. White-tailed deer are much larger than a hummingbird and it didn't take very much to kill the deer.
If anyone finds a hummingbird that is unresponsive, but still breathing, hold it in your cupped hands and breath on its chest with your warm breath. If it is not too hypothermic it will warm up and recover in about 15 minutes. It is best to do this outside in case the bird flies out of your hands. If it did that inside of a house, it would likely kill itself trying to fly out of the window and hitting the glass. If you take it in your house, you need to put it in a box (with small holes punched in the sides to provide air) on a soft cloth, close the box so the bird can't escape in your house and set the box under a light to warm the box. Make some sugar water at 1 part sugar to 4 parts water and nothing else in it. Let the sugar water cool to warm enough that you can feel that it is warm but not hot. As soon as the bird begins to flutter around in the box (don't let the box get too hot) it can be fed. Fill a medicine dropper or a small syringe with the warm sugar water. Take the box outside and open it. Hold the end of the medicine dropper or syringe at the end of the bird's bill, so it can slurp out some warm sugar water with its long tongue. Let it eat as much as it wants. Close the box and wait for about 10 minutes. Then open the box (outside of course) and feed it more if it doesn't immediately fly away. After it eats again if it still doesn't fly away after several more minutes and/or more sugar water, close the box and call the nearest rehabber, because there must be something wrong with it besides being cold.