If you have water coming out of the hot taps but it is cold, this indicates that you have either:
ELECTRIC: a blown element, a faulty thermostat or your cylinder could be wired on an old “ripple” switch so that the power company can control to turn off the electricity supply to the Hot water cylinder without you knowing.
LPG: indicates either run out of LPG or no power to the unit. If this is after a power cut, you may need to reset an instantaneous hot water system by switching the power off and on again. The instantaneous gas hot water system should then go through a reset programme and be ready to use again in a few minutes.