hot water pouring out of my overflow?

hi wondered if any one can help heard a massive amount of banging coming from my loft/airing cuboard today and then water started pouring out of my overflow ,havnt used the heating for a while and it started not long after being switched on, turned everthing off but the issue continued and the radiators were piping hot and the boiler sounded like it was still on, checked the heating tank in the loft and the water was boiling hot ,opened the airing cuboard and the mid position valve was red hot so using the toggle switch on the side i moved it over and it seemed to stop ,i let everthing cool down and attempted to restart the system but now the bolier won't start as anybody have any idea i'm abit lost