It would appear that someone did something wrong or didn't do something they should have done, but the root of the problem has to be at the water heater. A gas water heater should heat a tank of cold water in just a few minutes, not hours. I'd start with making sure the gas is turned on. If it uses a pilot light, it must be lit. If it uses an electronic ignition, then it must be plugged in. In my opinion, you should not have to pay the plumber if there was something he did or didn't do. If there is something wrong with the heater, that would be a different story, but I would not advise you to do more than a visual inspection of the heater.