Unlit pilot light.
Pilot light won't keep burning.
Clogged burner
Defective thermocouple.
The gas is not coming out.
Remedies:
Carefully relight the pilot.
Make sure the gas controls are completely turned on. Then check the thermocouple and be sure it is firmly connected to the gas control unit and positioned near the pilot flame.
Call a qualified contractor.
Replace the thermocouple.
Call the Gas Company.
Thats really weird, don't mess with it too much. If its not working, I would turn the gas off and get someone to check it. Don't want to blow yourself up. It's been so long since I had a Gas water heater.