The other possibility is that Google just updated their search algorithm in a way that happens to not like this website as much as it used to. Not because anything is wrong with it or because of any bug in the algorithm, just it now prefers different attributes, and that changes where this site ranks. If this is the case, there's really nothing to be done. There's no ground truth result ranking for any particular query - if Google feels its new results are better, that's their business.
If your entire business depends on an arbitrary algorithm preferring your website over other very similar ones, and you will lose everything if that arbitrary algorithm happens to change, eventually it's going to happen. It sucks, and it's maybe unfair in a cosmic sense, but there's really nothing to be done.