The one shot thermal trip on top of the heater tank is likely tripped. Cannot reset this, its a safety component. It has to be replaced with exactly the same part .. contact Redring to buy a replacement. First though check that the heater tank is not furred up with limescale, if you are hard water the flow may have reduced to such a low level and the tank overheated
If so it might be better to replace the shower.
Garry K
June 2007