No Hot Water At All: What to Check, in Order

First, which kind of heater

The fault lists barely overlap, so this is the question that has to be answered first. A metal flue rising from the top of the tank means gas. No flue, and small access panels on the side, means electric. On the gas or electric page there is more on telling them apart.

If it is gas

Total loss of hot water on a gas heater usually means the burner is not lighting. The pilot may be out; the thermocouple that proves the flame to the gas valve may have failed, which shuts the gas off deliberately; or the gas control valve itself may be finished.

A pilot that lights and then dies within a minute of releasing the control knob is close to a signature of a failed thermocouple.

If you smell gas, do not troubleshoot. Leave, and call your gas utility from outside.

If it is electric

Electric heaters have no flame to lose, so total loss usually means power or a safety cut-out rather than a burner. Check the breaker first. Then look for the high-limit switch behind the upper access panel — a small red reset button that trips when the water has overheated.

A high-limit that trips again after resetting is telling you something real, usually a stuck thermostat. Repeatedly resetting it is not a repair.

If it is oil, or there is no tank

In fuel-oil regions many houses make hot water indirectly, off the boiler rather than in a heater of their own. If your hot water disappears but the house still heats, or it fails only in summer when the boiler is otherwise idle, the fault is likely at the boiler.

What this rules out

If the tank is producing no heat at all, sediment is not the cause and the anode rod is not the cause. Both of those shorten a heater's life and degrade its output; neither stops it dead.