Your cat may have caught it and brought it home. Grass snakes will often feign death to escape a potential predator, and they can also give off a terrible stink to deter anything holding it. We once had a cat who brought home a much smaller grass snake, and I thought it WAS dead so I put it on a garden table, only to return a short while later to discover it had disappeared! It had been lying absolutely motionless, head tipped over, and no reaction when touched.
Well done for catching it, and thanks for sharing your
amazing story and photo with WAB.