Opal Triton shower will not start?

I look after a disabled lady. Recently she asked me to fit a new shower head to her Opal Triton shower which I did. When I turned the dial to test it nothing happened. I notice that there is a stop button but no start button, but she tells me thats all you have to do to start it, turn the dial. I asked her when she last used it, and she said about a year ago, but the old head sprayed everywhere, so she has not bothered since. Am I missing something here. Can anyone help please? Regards to everyone