Both sentences are seeking the same information, but the change to "did happen" from "happened" uses the emphatic form. In speaking, one would emphasize the word "did". This can be used to ask a question that contrasts with a previous explanation. Look at these examples:Originally Posted by Mazen
1. John is coming out of the boss's office and he looks very sad.
Mary: John you look terrible. What happened in there?
John: The boss fired me -- for losing the Johnson account.
2. John is coming out of the boss's office and he looks very sad.
Mary: John you look terrible. Did the boss yell at you?
John: No, that wasn't it at all.
Mary: Well, what did happen in there?
John: He fired me.