This used to happen on occasion to a big male koi angelfish that I had. He would jaw-lock with his mate (she was crazy) and I'd come home to find his mouth protruding like he was yawning. Usually it would only last a day or so, then I'd come home to find it back to normal, and he'd happily eat and go about his business.
Eventually this happened and it didn't right itself. It didn't look any different than usual, just protruding, but he swam around for four days and it never fixed itself. I attempted to gently push on it a bit when I caught him asleep one night, but it didn't correct the problem and he still had a protruding mouth. After a week or so of this, I came home to find him dead at the bottom of the tank. It wasn't enough time for him to starve, so I'm guessing he either succumbed to stress or the female finished him off.