It sounds like you added the yogurt to the milk and then heated to 180°? If so, you killed the bacteria in the yogurt. You should heat your milk + half & half to 180° and hold, and you will need to stir pretty much constantly or else it will scorch on the bottom. Then put the pot into a sink of cool water and cool it down to 120°. THEN add the yogurt, and then, if possible, hold it at 115-120° for 8-10 hours.
Unless you have a pilot light in your oven, it is likely it will not stay warm enough. Another option would be to wrap the pot in a heating pad. Best of all would be to use a yogurt maker -- you can get them quite inexpensively, and even a very basic one will work. And a hint - I have made 1 gallon of yogurt at a time using a basic yogurt maker, even though it was designed to make six cups of yogurt. I found a glass jar that fit, cover the whole thing in two or three towels, and voila!
More recently I've made yogurt in my InstaPot, which has settings for heating the milk to 180° and then for holding at 110-120° for however long you set it for.