When the reset switch goes off on an electric water heater, it generally means the element or the thermostat has gone bad. Elements can be tested by removing the wires from the element and putting an ohm test on the leads. Testing should be done only by a qualified service technician. You should not attempt to work with high-voltage unless you are familiar with all of the proper lock-out tag-out procedures and are qualified to work with electricity, as it can cause death or serious injury.
Both the element and the thermostat should be replaced at the same time, but first consider the age of the water heater. The life expectancy of most water heaters is 15 years. If the water heater is more than 10 years old, the labor cost of replacing the elements is about the same as the installation costs of a new electric water heater. You will pay for the water heater, but you will recoup some of the loss in energy savings over the next 10 years.