Translation:Tell her what happened this morning in front of your place.
I read a post from a French person recently who said that "se passer" is used most often. "Arriver" is more likely to be used when asking what happened on a more personal level, eg if someone has a broken arm and you're asking how it happened.
Again, someone please correct me if I'm wrong.
Nope, this is an entirely different contraction!
To happen = se passer. This is a reflexive verb; the se is part of the verb.
s'est passé is just a contraction of se est passé.
Unfortunately s'est and c'est are pronounced exactly the same, so you have to understand the grammar to know which one to write.