They're both correct. The second is the emphasised form.

what did happen to you is for emphasis but no one would really use it, it is awkward. Instead you could emphasise the you. What happened to YOU?

What happened to you? - is correct. This is a subject questions - because the question word is the subject of the sentence. There is no auxiliary verb and the word order is not inverted.

"What happened to you" is more common phrase and probably the one you want, a simple question about a series of events. "what did happen to you" the emphasis is on DID impliying that whatever happened is secretive or misconstrued and you are trying to get the truth

Thank you!!!!