Two important factors to check when troubleshooting a water heater are that it has power and that it has adequate water flow. Because of the combination of power and water in these devices, it is important to ensure that care is taken at every stage of the troubleshooting process.
Check that the heater is plugged in, if it is electric, or that the valves are open and the pilot light or igniter is functioning if it is gas-powered. If there is no obvious problem, continue to check the wiring and plumbing for leaks, breaks or other power interruptions. It's also worth making sure that the house's breaker has not tripped, and that the heater's desired water temperature is set correctly before going further with the troubleshooting process.
For an electric heater, the next thing to check is the elements. A voltmeter is a useful tool here to see if current is passing through them. If not, replacing the element may solve the issue. Don't forget that they need to be grounded to work properly.
For any type of heater, checking the water pressure against the rating of the pressure limiter is useful. Both too much and too little flow activate safety precautions and prevent the heater from performing effectively.