My MX Duo LXi shower wont turn off!?

I have a MX Duo LXi electric shower in my rented flat. It has been trouble free for 4 years but today decided to stay on when I turned to dial to off. I turned the power switch off in the airing cupboard which normally cuts the water, which cut the power but the water stayed on. I have had to turn the water mains off to stop the flow. I have tried to call my landlord but he is a pain to get hold of, is there an easy fix I can do myself? I have had a look inside the shower but have no idea. Any help would be greatly appreciated.