چه اتفاقی افتاد؟

The correct one is: What happened?

"What was happened" is wrong because the verb "happen" is intransitive. For transitive verbs you may use this structure: Wh + be + p.p. (The answer of this question is the object!)

• What was eaten?
• What was closed?
• Who is hurt?
• Which one was chosen?

When the question word is the subject of the sentence, you do not add do/does/did.

• What did make you cry?  >   What made you cry?
• Who does like apple?      >   Who likes apple?
• Who did go there?           >   Who went there?

When it's the object, you should use do/does/did:

• What did Ali tell you?
• Which one do you like?
• Who did you talk to?

what happened ?