The water in the raiadtors is the same water going round and round, the shower and hot taps use water from the hot tank (which got to the tank via the cold tank and the mains coming into the house)
Do you have water coming from the cold taps? If not, the problem is either outside your house, maybe a burst main somewhere, or inside before the tank. Look into your cold tank, you will see which.
If you have cold water but not hot, the problem is in heating the water (boiler? immersion thermostat? thermostsat kaput?) or in letting it get to the taps and shower (valves?)
If