Electric boiler problems

[cgh31]

This may make me sound ridiculous but......I have an electric boiler which usually heats the water when I turn on the socket at the mains. It now appears to be broken. When I flick the switch I can hear a hissing sound from the heater (poss the filament?). Does this mean the filament is broken? Can I replace this on its own or will the whole boiler need replacing? How much do these things normally cost and more importantly who do you get - a plumber or an electrician??!!!! Thanks in advance. x