If your water heater is an electric water heater try checking first to see if the breaker to the water heater has tripped.
If it has shut the breaker off then switch it back on. If it trips again then something is shorting out between your water heater and the breaker panel.
While the breaker is off open the access panel on the water heater usually the top one and look for a red reset button on the thermostat.
This button will shut the water heater off if the water heater gets too hot.
Push it back in and then turn the breaker back on and listen to see if you can hear the water heating back up.
If that doesn't fix the problem then your thermostats or your heating elements might be bad.
If it's a gas water heater try checking and see if the pilot light is out. If your pilot is out and won't light then your gas valve "the thermostat gas valve" on the water heater might need replaced.