It is common to say "What happened to him (or somebody)?" to inquire about somebodies' whereabouts. But is it right?
In my opinion, it should be "What was happened to him?" or "What did happen to him?", even it can be "What had happen to him (before today)?" Am I wrong in this understanding?
Happen is the infinitive/present form of the verb; happened is its past/past participle form.
So What happened? is the form you would normally use to ask what occurred in the past.
What was happened? would be a past passive and can't be said in English, unless you're inventing a new transitive use of happen.
And What had happen? is not a possible construction; you must ask What has happened? (if for instance you are arriving at a scene of confusion) or What had happened? (if you are trying to find out what happened before some event in the past).
What did happen is possible; you would use this if people have been denying that this, that or the other happened and you want to steer the conversation away from what did not happen.
As for why is "What happened to him?" instead of "What did happened to him?" or "What did happen to him?" the answer is given in a question I asked on EL&U: "Who turned off the lights?"
As for the tense to use, the OALD has the following examples for "happen to somebody/something."
I hope nothing has happened to them.
It's the best thing that has ever happened to me.
What's happened to your car?
Do you know what happened to Gill Lovecy?
The used tense is always the simple past, or the present perfect.
