Gas hot water systems are generally reliable and long-lasting water heaters, but that is not to say they do not come with their own problems every now and again.
If you have no hot water, the first thing to do is check the main supply of gas in your home. You can do this by going to your cook-top and seeing if it lights, if it does then the issue is definitely to do with the hot water heater and not the gas supply.
If the cook-top does not turn on you should check that the gas meter valve is turned on, if it is, call your gas provider. If the gas is turned off, turn it on and then get rid of the air in the gas lines. To do this, light your cook-top and wait until all the burners have a solid flame around them, then turn them off. Your hot water will be ready as soon as it has finished heating.
If the problem lies in your hot water system, you should firstly ensure that the gas valve for the hot water system is turned on. You can also check that the continuous flow is plugged into the electrical socket and if you have a controller, check that the temperature is set to the Australian standard of 70 degrees Celsius or 158 degrees Fahrenheit (internal temperature).
The pilot light should always be on, to take a look at this you will need to get down to ground level and look at the very bottom of the water heater. If the pilot light is switched off there should be instructions to turn it on at the back of you're inspection panel.
The first cause for this problem could be the distance between your taps and the hot water system. Obviously, it can take some time for hot water to travel from the tank to the tap. A good way to test this is to let your tap run for a few minutes, if the water does eventually turn hot then distance is the issue. A solution for this is a recirculating pump or insulating your pipes.
Another cause may be instantaneous hot water heater flow rate. Gas hot water systems need moderate to high water flow to ignite the pilot light. You can test if this is the problem by turning your taps on to a high-water flow and allowing them to run for a few minutes. If the hot water kicks on, then the problem is to do with the rate of water flow and not the heater.