happen是不及物动词,没有被动语态,所以what was happened 是错误的,把was去掉变为what happened ,表示过去发生什么.比如：What happened to him yeaterday?
what happens 表示一般现在时的动作,发生什么了?
what is happening?指现在进行时,正在发生什么事?一般现在时的动作和指现在进行时是什么东西呢？