This is usually caused by the heating element rupturing and shorting to the boiler/casing. You can test for this by removing the top/back of the machine (6 small phillips-head screws) and checking for continuity across the heating element contacts (the two 90 degree prongs sticking out of the top of the boiler). If there's no continuity then the boiler needs to be rebuilt with a new heating element and boiler gasket.
Additionally, a heating element rupturing is usually caused by over-heating due to mis-use, so before putting the money into repairing or replacing it you might want to make sure you're using the machine properly. Regular descaling is also key to the boiler's health.