My old golf had this. They'd found themselves disconnected from the centre adjusting shaft of plastic. I have no idea how this would come to be really. But I managed to get them to click back in simply by wiggling my fingers in between the horizontal shafts the vertical ones behind. There was a little bit of bending and give in the vent flaps though and I have wiry piano playing fingers :huh: .
The accord has chunkier vent grills so I imagine you need to get something like some tweezer pliers involved in the job. However, you don't want to go scratching the plastic with the pliers. Maybe wrap some electrical tape around some small tweezer pliers so the metal doesn't scratch the plastic, slide them in between the vents, grab the problematic bits and try and click them into place maybe?