Learn Hardcore Serbian: Šta se desilo?

So se is not being translated as itself in normal English. This is one of those Serbian verbs that you should learn together with se as a single dictionary item:

desiti se = to happen
nadati se = to hope
smejati se = to laugh

So Šta se desilo? should be understood as a whole, not word-for-word as What itself happened?

What grammar form is desilo?

Desilo is the past form of desiti se.

More precisely, it is the past participle form used to make the Serbian past tense. Serbian often uses this type of past form where English would simply use happened.

So:

desiti se = to happen
desilo se = happened

Because the verb is perfective, it refers to a completed event: something that happened as a whole.

Why is it desilo, not desio or desila?

Because šta is grammatically treated as , and the past participle agrees with it.