only cold water comes out of the tap when the hot faucet is turned on.?

Check the circuit breaker or fuse first. It might be bad elements or thermostat. How old is it? If it's more than 8 years old I would replace the whole thing rather than just the elements. Replace with a high efficiency model and replacement will pay for itself in a few years.

Cold Water From Hot Tap

Depends on wether its a electric or gas. If its gas then the most common problem is the piolot light went out. You would have to relight it. The piolot light is inside the water heater at the bottom and there will be a little tin cover. Most of the time you have to lift up, then out to remove the cover. Then turn the knob on the gas valve to "pilot" and push the knob down. Then take a match and relight it the pilot. If its an electric water heater then it could be a few things. Most common problem is the breaker is blown or a heating element is out. To replace a heating element you need to shut off the water leading into the heater, turn the breaker off (very important to shut off the breaker) Then drain it. On a 40 gal or more water heater you will see two seperate covers. One at the bottom and another about midaway up. Inside each cover ther is a heating element, and thermostat. any of these could be bad. If you don't have a Volt/amp meter it is difficult to test any of these. If you do have one, then you would ohm out each element to and thermostat. If an element is bad you will eitther have to get a lerge 1 7/8 socket to unscrew it or you will have four little bolts to remove it. Mske sure to shut off the power to the unit before draining it and be sure to drain it before removing elements.

Heating element is bad.You could have two elements depending on the tank.Change both.Drain tank and make sure you suck out all the lime deposit built up in the bottom of tank.Make sure you label the wires leading to element.They make a tool to remove element with.Once you replace the element you should have hot water.BTW turn off breaker to tank.This is for electric water heaters,gas check burner.

check your prit light in your hot water tank to see if it is on. if not light it . if it don,t light, then you need to change the tholmo. coupling ,so it will light . if it a elec tank check your broker to see if it tripe. if not then one of the heating element as went . and that have to be check out.

check the tank and see if the pilot light is on first

heating element if its just luke warm, thermostat if it was to hot then none. breaker if it set for awhile and wasn't used

Water heater is probably bad. costly to repair.