Right below the thermocouple if applicable
Gas not turned on to water heater. Gas valve on water heater not set to pilot setting. Faulty gas valve. Igniter for lighting pilot not working.
Gas water heater has an orifice for main burner and for pilot light, if not electronic ignition.
No because water heaters have a safety valves and when the pilot light goes out the safety valve shuts off the gas
THERMOCOUPLE or defective gas valve
small amount
Same as any hot water heater. Turn the gas valve to off.
gas valve and pilot assebley cost more
If it is gas operated, the hot water heater may not light because it may have a defective millivolt sensor. The millivolt sensor is heated up by the pilot and generates a fractional amount of voltage telling the gas valve that the pilot is lit and the gas valve can now be safely opened to heat the water heater.
No, it is not dangerous. (Unless you are working with a water heater that is 20+ years old.) In any modern hot water heater, if the pilot light goes out a fail-safe will go off - preventing gas from leaking out. Also, there is a sensor so if there is not enough air-flow or oxygen with the water heater, it will shut off the gas.
A small amount to keep the pilot light going.
low back, spillage, or a bad thermocouple.
Yes, gas line needs to be grounded.