Is it more correct to say What happened or What's happened?

Either is correct depending on the sentence:

What happened at the party? What's happened at the party.

What's happened to the cookies? What happened to the cookies.

~Note: This is because the word "what's" is a contraction of "what has", so the sentence really means, "What has happened to the cookies?" That's a different tense than past tense, but either can be used.