Turn your water heater off,and do as previous poster has said, open cold tap,wait til water flows freely . Do same with hot tap. Turn water heater on,and wait at least 10/15 mins before drawing hot water, it needs time to heat up ,it's mot instant like your house boiler can provide.
After about 5 mins with the heater switched on, check the hot water tank with your hand, see if it's warming up. If it is, you're ok,if not check the fuse ,hopefully thus will be the answer,and you haven't burnt the heater element out through heating the tank dry.