Electric water heaters are fairly simple. Power in should always be the 220vac. It goes to a thermostat, then to the heating elements. Have you tried to find the manufacturer's instructions on their website. It should have some troubleshooting, a parts listing and some pictures. Three things commonly replaced: heating elements, thermostats, and leaking T&P valves. Once in awhile, people will think to replace the anode rod before it is totally toast, and if they do, the thing might last longer. The thermostats typically only activate one heating element at a time.
Assuming the more common wiring, they used a cable with black, white, and ground. There is no neutral on a common domestic WH, so between the black and white, you'll have 220vac. If you don't at the WH, you've got a problem. If the CB is on, and you don't have it at the WH, either the breaker is bad, or the wire is open.
You could have a bad thermostat(s) or burned out elements. Turn the power off, remove one lead from the heating element and check the resistance (Ohms) between the two leads of the element...it should read very close to zero. If it is open, the element is burned out.