why did you "suddenly switch it on" lol
sorry i shouldnt joke with newbies (or otherwise).
right, firstly, turn off the shower circuit at the consumer unit or fusebox.
grab a screwdriver and open up the shower switch, gently pull it away from the wall.
Examine the wiring inside. If there is signs of scorching, burning, melting, green copper (green is rusted), or if the back of the switch has any of the above, do not use the shower until you seek further advice on here.
If it looks ok, and theres no signs of the above, then you may just need a new switch. They do fail, due to the large amounts of currents used by showers.
