I'm not exactly sure how it happened, but Saki's Sword has fully short-circuited. No matter how much stamina I have, it won't swing. I've run into this problem before, early in my playthrough, and after not using it for a while it managed to fix itself, but I've run into it again and I'm at a bit of a loss. I think it might be caused by attempting to swing the sword while out of stamina? You could probably replicate the bug by spamming the attack, it seems easy to fall into even if you aren't spamming it.