happen to 后面跟的是人名、公司、或其他集合名次和代名词
What happened to you?
It happens to everybody.
The same thing happened to John and his family last week.
Please make sure this doesn't happen to us again.
happen at 后面跟的是时间、地点等状词,补充说明发生的那件事情.
The accident happened at near the corner of Main Street and Hill Avenue.
The recent earthquake in Janpan happened at 1:48pm the day before yesterday.