First, please don't start your question with 'I wanna'. I know that you don't intend to be impolite, but it does give the impression to native speakers that the writer is rude, impatient and childish. Please write 'I want to' or, more politely, 'I'd like to'.
I'm guessing that your confusion about the question 'What happened?' is that it looks too short. Why is there no 'did' or 'have' or 'is' in the question? Well, we don't always need these auxiliary verbs. Here are some more questions in the past simple with no auxiliary verbs:
What broke?
What fell?
Who won?
Who phoned?
All of these questions, just like 'What happened?' are formed simply of two words - the question word and the main verb in the past. And the reason that we don't need an auxiliary verb is that 'What' or 'Who' is the subject, not the object of the question.
You hear a crash, and you know that something fell off the table. You say 'What fell?' You know that someone phoned, so you say 'Who phoned?'. You know that something happened, so you say 'What happened?'.
If you want to use the present perfect tense, for a very recent event with results in the present time, you can, as you say, ask 'What has happened?' .
The 's' in the short form 'What's happened?' is 'has'. It is the same 's' form as 'Where's he gone?' (Where has he gone?).
And yes, you are right, you can never use 'is' with a 'ed' form. 'What's happened?' is 'What has happened?'