The comments are correct; this statement is correct as is and should not be in the passive voice.
We only use the passive for transitive verbs, ones with an object, where you can say "X verbs Y". For example
The cat eats the fish.
The fish is eaten by the cat.
But it doesn't work for intransitive verbs like happen or fall or smile, because they don't have objects:
The boy smiled. (no object)
Incorrect: The boy was smiled
"The boy was smiled" would mean that there was something else that performed the action of smiling on the boy, but the verb to smile doesn't work that way.
Likewise, to happen is intransitive:
The event happened. (no object)
Incorrect: The event was happened.
Happen simply means "to occur or to take place", and the event that happens is the subject, not the object.
Intransitive Verbs (Never Passive)