I have a gas hot water heater and it come on but when you turn on the hot water cold water comes out could you please tell me what's wrong.

It just happened, how old is your heater and there is no hot water any where in your house? check your heater gas valve for proper setting see the image below your gas valve should be set at Aprox 120 degrees, if that set and hot water get back to us Thanks.

John

In addition to Afaroo's advice: It may be that the combustion vents, located along the bottom of your heater, are obstructed. As the heater activates, demand for oxygen rises. Unable to draw enough oxygen to burn, heater returns back to pilot only mode.