What happened" = is used to inquire of a specific incident in the past. "What happened to the turkey I cooked?" (it was eaten) "What happened to the people who are in line?"(they got tired and fell asleep) "What's happened" or "what has happened"= is used to inquire of a status which brings us up to now. It is more general than specific. "What's happened to the turkey I cooked?" (I am looking at it, it is nothing but bones, we can assume it HAS been eaten) "What's happened to the people who are in line?"(I am looking at them, they are asleep therefore I can assume they got tired)