2 seperate issues, first the water heater, have you got a system with a removeable cover on the outside, if so the cover need to be removed for gas to operate, secondly is it the oven that is not lighting or the hob or both. on some ovens the lid needs to be raised to its highest position for the gas to work, also in both cases you need to check the tap in the gas line to each device is turned to the on position, you will normally find the taps in a cupboard in the kitchen area, do you have gas working on anything else, such as fridge or barbecue point,and you also need to check you have gas in the cylinder in the front locker, and that it is turned on