The most likely fault is that the high temperature thermal cut-out has failed, this usually a non-resettable device. If the shower unit has been in service for a number of years, the cut-out has probably been triggered by the heat build up due to the accumulation of lime-scale, the cut-out usual blows when the shower has been run on the high setting and suddenly turned off. A continuity test of the cut-out will confirm this if open circuit. The cut-out and heater tank will need to be replaced to prevent a re-occurance, However, at current prices it is usually more cost effective to replace the complete shower unit.