Blackwall disappeared from Skyhold

I had the same problem where Blackwall disappeared from Skyhold. I'd talked to him several times there before and finished his side quest. I wasn't able to find any letter by searching and shutting off the game and turning it back on didn't work. I tried someone's suggestion of saving in the barn and reloading and suddenly, he was back in his usual spot. I triggered two more cut scenes with him (the last one was him getting a drink with the Inquisitor and talking about a dog) and then my search indicator popped up in the barn showing me a letter pinned to the rocking-griffon he was making. Then, within seconds, a messenger showed up with a note for me and confirmed he was gone. I selected his inner circle quest and that's how I got to his special scene where you learn more about him.