Open the water tank top panel & hit the reset button if it is not accessible from the outside.
Test the wirings to determine if the water tank is getting power or not.
Call the service center once the steps above prove that it is the fault of the tank & is not from faulty wirings or breaker.
Replace breaker and test the wirings lead if the breaker is the problem.
Follow the steps below if it you are a DIY.
Turn off the power and shut off cold water line at top of tank.
Do a continuity test to see if the elements are the problem (remember to undo the wirings before testing).
Turn on hot water taps in your home & leave it on.
Drain the water by hookup a garden hose to the sediment valve at the bottom of the tank.
Open the Pressure & Temperature Relive Valve at the top of the tank (becareful because hot water can spurt out and burn your feet).
Visually inspect the first gallon to about 6 gallon of water to see if there are sign of sediment/rust (a little rust or discolour water is normal, replace tank if discolouration after 6 gallon of water has drained).
After 1/2 hour of draining water undo the top element by un hook the wirings (mark the positions of where each wire should go), then undo 4 screws (flange mount) or turn the element screw counter clock wise to unscrew with a hot water tank element tool (screw mount, and the tool can be found for less than $5 at your local hardware shop).
Use a deep socket ratchet wrench or square head wrench to undo the anode/s nut at top of the tank (the nut/s can be access easily & easily seen, others are hidden under a plastic or metal caps some water tank anode/s nut and are under some insulation. Some have only one anode at center or side of tank, and some have 2 anodes).
Visually inspect the elements to see if the top element is the problem; also try a continuity or resistant (ohm) test.
Follow the above procedural for bottome element after the rest of the water has drained (additional 1 hour or more to drain water level below second element, and it depends on tank size).
Element replacement are cheap that can be found at your local hardware shop (most element are 4500W, a few are 3000W).
Replace the anode with appropriate type (aluminum, or magnesium) for your tank if there are sign of excessive corrosion (calculate the life of an anode by visually inspect to see how much metal is left?anode rod should be replace between 2-3 years of use)
Element & anode threads should be wraped 1.5 or more times with Teflon tape to make sure you have a tight seal before you screw them back on to the tank.
Close the Pressure & Temperature Relive Valve and open the cold water intake valve at top of tank and wait till the tank is full (about 10-15 minutes and it depends on tank size)
ATTENTION! Maker sure the tank is full of water before turn the power back on (element will burn within seconds if it is not immerse in liquid)
An indication of a full tank is when cold water comes out of the tap in hot position after you turn on the cold water intake at the top of the tank.
Turn off all water tap and touch the hot water out going line on top of the tank to see if you have hot water.
Task is done if the copper line is hot to the touch.
Good luck!