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I presume here that you want your hot-water service replaced if this is correct then you need to engage a plumber to investigate your hot-water unit.
This may be a case of a tripped circuit breaker, in this case you should be able to see this at your meter box and simply try a reset.
It may also be that the heating element has failed, in this case the plumber will get an appropriate replacement part and install it.
Cheers Neil