If its the black plastic hose that was removed from the silver nipple then that shouldnt have been done? A special tool is available to remove the silver disc shaped washer from the rail itself, it is just really well sprung for security, you can make one up youself though.
It will just clip back over when re fitting if its lined up squarely, try moistening the rubber seal though to help them slide in the pipe. dont forget the wired safety clip over the top of that to stop it seperating.