Here, "what's" is the contraction of "what has". This is the present perfect, which is constructed using:
1. the helping verb "have"
2. the past participle of the main verb
The meaning is almost the same, but not quite. You would use "What happened?" to refer to an event in the past; "What's happened?" refers to an event in the recent past which has an effect on the present.
"I was expelled from school in 1998."
"Why? What happened?"
(Peter drops a tray of cups and saucers. Mary hears the noise from another room)
Mary: "What's happened?"
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