A leak.A safe first step.
Start with the danger check. Only take an action if the equipment and the route to it are dry and safe.
Water near electricity?
Keep clear of wet switches, lights and the consumer unit. Do not touch them or walk through water to reach them. Move to a dry, safe place and get a qualified electrician’s help.
Only if the consumer unit is dry and safe to reach
Switch off the immersion heater at its own switch or its clearly labelled breaker. Keep your hands dry. If the label is missing, the consumer unit is wet or you are unsure, leave it alone and ask an electrician.
Close a known, safely accessible stopcock
If you know your main water stopcock and can reach it safely, close it. Do not go into the loft, force a valve or approach wet electrics to find one. A plumber can isolate an unfamiliar supply.
Arrange help from a safe place
Contact a qualified plumber and your insurer or landlord. Keep away from a sagging ceiling. Do not drain the cylinder yourself or switch it back on before a professional says it is safe.
Do not restart or repair it yourself.
Leave water-damaged electrical equipment alone. Have a qualified professional assess the installation before electricity is restored. Do not climb into the loft or stand under a bulging ceiling.
Keep records safely.
Tell your insurer or landlord what happened. Photographs may help, but take them only from a dry, safe position. Do not approach damaged equipment or delay getting help for a photograph.