My mixer shower that runs from the combi boiler is severely leaking. Any advise on how to fix it?

My mixer shower was in use and then can't be turned off now. It is on for a good few hours but don't know how to stop it. Have stopped the mains water supply but need to get water back on in the rest of the taps. Is it very difficult to fix? Is it very difficult to install a new mixer shower? Much appreciate any tips and advise.