Meaning of “happen” - Learner’s Dictionary

happen

verb [ I ] uk us /ˈhæpən/
EXIST

A2 If an event or situation happens, it exists or starts to be done, usually by chance:

Were you anywhere nearby when the accident happened?
We can't let a mistake like this happen again.
RESULT

A2 to be the result of an action, situation, or event that someone or something experiences:

Did you hear what happened to Jamie last night?
What happens if we can't get enough tickets?
happen to do sth

to do something by chance:

If you happen to see Peter, say "hi" for me.
You don't happen to know her phone number, do you?
as it happens; it so happens

something that you say in order to introduce a surprising fact:

As it happens, her birthday is the day after mine.

(Definition of “happen” from the Cambridge Learner’s Dictionary © Cambridge University Press)