A water heater that produces no hot water may not be getting power, may have a tripped limit switch, or may have one or more failed heating elements.
First, check the water heater's circuit breaker in the service panel to make sure it hasn't tripped. If the breaker has tripped, switch it off, then switch it back on.
If the heater's breaker did not trip (if it is still on), try to reset the high-temperature limit on the heater
