No hot water can be due to a range of issues; Hot water cylinders devices have many important parts to ensure they produce safe tempered water and can heat with out increasing the pressure of the tank. If you have an electric hot water cylinder, there could be an electrical fault, the element may have blown or the thermostat may be faulty. On top of issues with the heating the safety valves can also fail, hot water tempering valves that ensure the hot water cannot burn you will fail in a ‘cold’ position, meaning the water may be heating but not being allowed to get to the taps.
Gas cylinders can have similar issues with valves and safety measures, in some cases the pilot light that ignites the gas to heat the water can blow out, stopping the flow of hot water as the tank cools from lack of heat.
Hot water devices are complicated, therefor it is always best to have a professional take a look at the issue first.