My companion had the mask equipped and when he joined the blades, he changed his outfit. So I looked at his inventory to take the mask back, but he didn't have it anymore. I tried everything, but I think the mask glitched off. It's either recover the saved data or start over since I can't progress through the game anymore.
This is a known bug:
If any mask is given to any follower, over time it may disappear from their inventory.
So, as you stated, your options are limited to continuing from an earlier save point, or start over.
The good news is, this is not plot essential; it is only required in order to gain the Konahrik mask, so you can continue on with the game.