On electric water heaters they have a little red button on the thermostat that is there as a safety mechanism.
Turn the breaker off to the water heater and then open up the top cover with a screw driver.
Push in that red button and then turn the breaker back on. If that was the problem you should have hot water.
If not then your heating elements could be burnt out.
Those can be replaced if you know some about wiring. If not call a plumber and have them do it.
If your water heater is 10 years old or older I recommend just replacing the whole water heater as it could be on the verge of leaking.