This can happen ('cos I've done it - more than once) if you pull the grill out to change a headlight bulb, say, and then you plug the grill back in without ensuring that the plastic release strap is lying on top of the grill (and not stuck behind it) before closing the bonnet. Just takes a bit of fiddling with a hook of some description to pull the plastic strap back over the top of the grill. You'll do it in a couple of minutes. It might even be possible, there may just be room, with the bonnet slightly up on its primary catch, to lift the grill upwards and out of its slots. But then again, there probably isn't!

Not the end of the world. Not right difficult, just a fiddle. And like me, you'll probable do it again, next time.