Certainly sounds like something has tampered with the box. I definitely wouldn't try to reinstall nesting material though.
Sounds like it could be Jonathan, was writing to someone with the same type of problem,l don't know with the material, my gut feeling says see if the parents come back and put it back, do you know if there are eggs or young in there? yas
Hi Jonathan, does look like something has been at the box. If there are chicks it could have been a cat or a larger bird trying to get to them. Was the box secured enough in the first place, could it have been the parents? If no chicks and birds going back and forth, leave well alone and see if they sort the nest material. If you see nothing happening and no chicks and the box is loose, you could quickly tighten it, I am sure that would be okay.