Its a pressure release device and the most common cause of it being activated is because the hose is twisted or kinked badly enough to prevent the water flowing through the shower head.Isolate the electric remove front cover of shower usualy one screw holding it ,it maybe behind a plastic name plate or there are screws at the top and bottom of cover and gently re insert the rubber ball back in to the little entry or if you lost it get a replacement prd from the manufacturer and replace the shower hose and or head if its damaged.PUt it back together and run cold water through the shower with out using the electric if it allows you (my old shower was able to do this)to prime it making sure the temp dial is in the cold setting switch it off and try it with the electric on and temp dial on heat hope this is of help .If in doubt get professional help.
DIY ER in Carrickfergus ni, April 2010
