This is the 1735th most frequent German word.
geschehen
"Geschehen" means "to happen" or "to occur."
In this sentence, 'geschehen' is used in its past participle form to mean 'happened' in the context of asking about an event.
Was ist geschehen?
What happened?
Here, 'geschieht' is used as the present tense form, meaning 'happens', describing an event that occurs frequently.
Es geschieht oft, dass Menschen überrascht sind.
It often happens that people are surprised.
In this case, 'geschahen' is used as the simple past form to describe an occurrence in the past.