Your shower has overheated internally and requires a new thermal cut out to be fitted. your best bet is to contact a shower service enginer (if your very confident around power you could relace it yourself, it is a small flat device on top of the shower tank held down (usually) by a bolt the disc has 2 wires attached to it.)
It would have over heated either because the water was flowing too fast or too slow, the shower head must remain above the bottom edge of the shower unit (the hose must be in a U shape) otherwise the element will over heat due to the water passing over too quick, if your in a hard water area the head should be cleaned once a month or so using a standard descaler to prevent slow water flow
TD, October 2008
