There are many reasons why a water heater isn’t supplying hot water. One of the most common issues in gas water heaters is when the pilot light goes out. Many people can relight the pilot light without assistance but sometimes may need to call a technician for help. Electric water heaters may not heat water if the thermostat becomes faulty or the breaker trips. If resetting the breaker does not help, you may need to have the thermostat checked.