What happened was (that) the door wouldn’t open. The thing that happened was: the door didn’t open. Ce qui s'est passé était que la porte ne s'ouvrait pas. (Sorry if my French is not perfect.)

The context is probably someone is late or couldn't do a task due to being locked out or a jammed door. So the conversation was probably like this: "Why didn't you clean out the closet like I wanted you to while I was gone?!" "I was trying but couldn't get in! What happened was the door wouldn't open! Someone locked it, and I don't have the key!"