Gas or electric? Gas, the most common problem is the thermocouple being bad. If you have a pilot light that is on but the heater will not light, that is probably the problem. Easy enough to fix. Electric, check the breaker first. It they are on, it's probably one of the heating elements burned out. Probably the top one, but it can be either. These are also easy enough to change.
It's called water heater. There isn't a reason to heat hot water
Have you turned the control from Pilot to On? Have you increased the temperature setting?
It is a water heater, not a hot water heater. There would be little point in using a hot water heater. I assume that is your point.
Because heat rises - the top of the water heater will always be hotter than the bottom.
Because hot water heater breaker has tripped, or hot water heater has failed.
More than likely you will not have heat being the water wont be circulating through the heater core , When the hot water passes through the heater coil the air passing through the heater coil makes heat blow out the vents when you have selected a heat setting on your controls.
It's called water heater. There isn't a reason to heat hot water
Heat tape is just for keeping the pipes from freezing, it does not heat the water. That's what the hot water heater is for.
Electric hot water heater is used to heat cold water for use in winters. Electric hot water heaters heat water faster than the ones which consume fire in order to heat water.
Your heater core is likely bad
There are two insulated heater elements that heat the water.
Convection
In a hot water heater, a thermostat regulates the heat
Latient heat
THERMOCOUPLE or defective gas valve
Central heating systems usually heat on a different system from the hot water heater, unless the main heater is a combination furnace/hot water heater. If this is the case, it could be because the outflow of hot water is greater than the ability of the furnace to heat the water.
The part that generates heat... depending if it is a gas / electric heater