1 occur, take place, come about, go on, come to pass, develop; betide, chance, prove, materialize, Colloq transpire, come off, Slang cook:
What is happening? As it happens, everyone has already left.
2 befall, become of:
What will happen to me if I refuse?
3 happen on or upon. come upon, chance or hit on or upon, stumble on or upon, find, turn up, encounter, meet with:
It was then that I happened on an old letter of his.