Bee hive knocked over!

I looked out my window and saw my bee hive knocked over! I ran out but it was too late. All the bees were dead. I have no idea what knocked it over, there were no animal tracks or prints on the hive. It was not windy out. There had been times when it was very windy and it stayed up fine. So I don't know what happened. Now I have to start all over this Spring. I'm trying to figure out how to keep it from falling over again. Any ideas out there?