Literally: "what happened?" Comes from some Japanese video game in a poor English translation. See also: all your base are belong to us, someone set up us the bomb, make your time, et al.
An interrogative as to the events that have come to play, be it in the distant past, the recent past, or the present. This phrase may or may not use a question mark, but is an interrogative, regardless.
This phrase, popular among geeks, alludes to the poorly-translated Sega game Zero Wing; however, it is not always used for its pop culture value alone.
person: "Oh shit."
person2: "what happen"
person: "My term paper got deleted from my computer!"
person2: "Seriously? Oh, shit! Are you sure there isn't a back up somewhere?"
person: "Nope, not one back-up."
person2: "All your docs are belong to trash :("