A sparks will tell you to get a plumber. Check one thing first, turn off switch, remove cap from immersion (by the one nut holding it) and you will see a thermostat with numbers on it (to set temperture), beside this you may see a tiny reset button, press it in if it clicks then it tripped out for some reason and should now work away as before. Some immersions don't have reset buttons, depends on there age.
You will need a plumber to change the immersion and they cost about €32-€40 plus labour plus VAT.