What happened? = simple perfect (past) time aspect* What's happened? = What has happend = present perfect time aspect* The difference is very slight, but "What happened?" is a question about a past event that has already been completed. "What's happened" implies that the action happened in the past, but the situation is still present and possibly (not necessarily) still happening. *Note that we talk about these forms as "time aspects" and not "tenses," because a time aspect is slightly different than a tense in linguistic science.

What's happened is short form of what has happened. What has happened to your dress?There is a flaw on your dress. What happened? Refers to something happened in The past, it could be 2 minutes ago, or last week. What happened in Orlando on June, 12, 2016?