A functioning hot water system is vital to a modern household—and it can be a disaster when you turn on the taps to find only cold water comes out. If this happens, there are a few things you can check yourself before you call the plumber.
The first thing to check is the circuit breaker or fuse on your switchboard. If the circuit breaker has tripped, you can just switch it on again; if a fuse has blown, you can replace it with the correct fuse wire. However, this probably indicates some underlying problem with your system, which you will not be able to fix yourself.
If you have a gas system rather than an electric one, it may be that the pilot light has gone out. It is usually easy to relight it. The exact procedure will depend on your system, but there should be instructions written on the inside of the front cover panel. If you are not confident about following the instructions or relighting it does not seem to work, call in the professionals.
Pressure relief valve
Your hot water system has a temperature pressure relief valve. There will be a little lever at the top of the valve for testing the system. Lift this lever carefully to allow water to flow through the overflow pipe (make sure no one is standing near the pipe, as the water will be hot). When the water has flowed for a few seconds, you can carefully close the valve. The water should now cease to flow. If any water continues to flow from the tank, you will need to call a plumber.
If there is no water flowing from the hot water tap, the isolation valve may have been turned off. It is usually found at a lower level than the water heater inlet. You will need to turn it anti-clockwise to put it into the "on" position, and it may have an ordinary tap head or a black knob. If it has accidentally been turned off, turning it on again may fix the problem.
Trying to fix the system yourself can be dangerous and may be illegal. If the above steps have no effect, or you are in any doubt whatsoever, don't hesitate to call a local hot water plumber, who will be able to bring your hot water back quickly and safely.