I'm experiencing this myself right now. I left out yogurt for almost a full 24 hours in 80-90 degree weather. I had a lot of stomach pain and diarrhea for the day, but it seems to be getting better. Just drink lots of water, some carbonated beverages to settle your stomach, and eat healthier foods, they won't upset your stomach as much. For the most part I'm fine though, no vomiting or anything. Longer than 24 hours, I would suggest it.
You might get sick to your stomach, but its not very likely anything serious will happen, because yogurt doesn't spoil very quickly.
You shouldn't eat it if it has been left out for a long time because it is a milk product and you can get food poisoning. So DON'T EAT IT.