I checked everything out, and the light bulb worked and there was electricity to the end of the extension chord. So I brought the light in the house to trouble shoot, seeing what was wrong. I plugged it in to the outlet, and immediately the light fixture burst into flames! I had a small fire going so I unplugged it, and the fire died out immediately. Then I found another chord and light fixture to screw the bulb into, afterwhich I screwed a curtain rod holder into the middle of the chicken's ceiling. Then I wrapped all the excess chord around that rod holder, and now the chickens have a heat lamp again to keep them warm. Banti roosters are fiesty, and I thought he might try to fight me, but he didn't. He seemed to know that I was helping him. When the light came on, he went to crowing about it, and he and the hens went to scratching and eating, so they are happy again! I think it was some snow falling on the light fixture that caused it to short out, because it was too close to the door, but now it is in the middle of their ceiling, so it should be safer now!
