Sometimes yogurt doesn't turn out.
You reach into the warm oven in the morning and instead of firm tangy yogurt you have warm milk. Oops. It happens to everyone once in a while.
Before you throw away all that milk, here are a few things to try to fix it.
The problem is always either temperature or culture.
If your culture is/was dead, nothing you can do will make your milk turn into yogurt. You need to get a new starter and begin again.
Usually though the problem is one of temperature and is easy to fix. If your yogurt didn't turn out, you either put the culture in when it was too hot and killed it, which isn't likely if you were using a thermometer carefully, or, that it cooled off too fast before the yogurt could thicken.
The latter is almost always the case, and is relatively simple to fix.
Put your yogurt back into a pot with a thermometer. Stand over it like a hawk and make sure to remove it from heat the instant the thermometer reads 110-115F. Any warmer an you will kill your culture. (If you have more that's not a problem, but if you are like me and use the last of your former batch to start the next one, you probably don't any more culture handy.)
Pour the warm again milk back into the storage containers and put it back in the warm oven. If you have a digital temperature control and can set the oven to 110F do so and leave it on. Check on the yogurt in 4 hours.
If you have an older oven, set it to the lowest possible setting, just when the light comes on, and turn it off once the light goes off. In two hours check the inside of the oven to see how warm it is. If it's starting to feel cool, turn it back on again for a few minutes to warm back up. Be sure to turn it off after a few minutes. You don't want to kill the culture or cook the yogurt.
Most times this will do the trick and closer attention to temperature will help your yogurt work the second time around. If it doesn't work get a new starter and use this batch as buttermilk in recipes. I wouldn't try it a third time.
If you find that your oven just doesn't stay warm enough long enough here are a few alternate ways to incubate yogurt.
Alternate Methods of Incubation for Yogurt