Your hot water heaters pilot light is on but the heater wont come on?

The thermocouple is bad. The thermocouple is the small metal tube that sits in the pilot light flame and runs to the control valve. The thermocouple generates a very small electrical current that keeps the gas valve open. They go bad regularly.

I would have to say to check to make sure the valve is not in the "Pilot" position. If it is then turn it to the "On' position. If it still isn't working then you may have a bad Gas Control Valve. If the thermocouple was bad the pilot light would not stay lit. If the pilot light stay lit, it is doing what it is supossed to do.