There are some problems with water heaters that you can fix without calling a plumber. If the water is too hot or not warm enough the first thing to do is check the thermostat settings. You can adjust it up or down to suit your needs.
If you have a gas model, check to see if the pilot light is out. Fluctuations in the main gas line to your house can sometimes cause it to go out. Follow the instructions on the side of the water heater to relight it.
Electric water heaters are 240 volt appliances so shut the power off at the main panel before doing any work on it. A shock of this size can be fatal. If you have any doubts about what to do, call in a qualified plumber.