Start with the cylinder's own thermostat or programmer, if it has one, and check it shows the right settings. After a long power cut, the timer or programmer clock may need resetting to the correct time. There is no loft tank to look at, because an unvented cylinder connects straight to the mains. It runs on mains pressure too, so if the pressure coming in is low, your hot taps will run weak.
Next, look at the tundish. It sits near the cylinder's pressure relief valve and lets you see when water is escaping from it. A drip through the tundish points to too much pressure or a faulty relief valve. If water is flowing through it steadily, call a heating engineer straight away.
Check the overflow pipe from the tundish is clear as well, because a blocked pipe can let pressure build up. Past those checks, leave the cylinder and its safety valves alone. Only a qualified professional should install or maintain them.
A yearly service is worth keeping up, because the makers usually make it a condition of the warranty. If the settings look right but the hot water still has not come back, or the tundish is dripping, give me a ring. At G R Plumbing I fit and look after unvented hot water systems across Caterham, so I can come and check your cylinder and its valves.
