How high off the floor was the belt? It is permissible to get on a moving belt if it is less than a few feet off the floor. If necessary you can walk on "gathering" belts for example.
If the belt was only a foot or so higher than that, you could argue that your offense wasn't all that bad, and the suspension was an overreaction, and possibly didn't follow the progressive discipline proceedures.
It's also worth noting in hindsight that you were not, in fact, injured.