There is no hot water.
Check that the cold faucets run normally first. If they do, the supply is fine and this is the water heater. Then check the breaker on an electric heater, or whether the burner is lit on a gas one, and whether anything has been left switched to a holiday or vacation setting.
Most no hot water calls end at one of those three.
Work through it in order
1. Is the cold fine? If the cold is weak too, it is a supply problem rather than a heater one. Start there instead.
2. Electric heater: the breaker. Water heaters usually sit on a double breaker. Push it fully off, then on. If it trips again, stop resetting it and call an electrician: a tripping water heater breaker usually means a failed element or a shorted thermostat. Why repeated resetting is the wrong move.
3. Gas heater: is it lit? Look at the burner window. If it has gone out, follow the relighting instructions printed on the unit itself, and only those. If it will not stay lit, or lights and goes out repeatedly, that is a thermocouple or a gas valve and it is a technician.
4. The setting. Check for a holiday or vacation mode, and check the thermostat has not been knocked down. On a tankless unit, check it has not locked out with an error code, and write the code down before resetting.
5. Timers and off peak tariffs. Some heaters run on a timer or a controlled tariff. If the hot water arrives late in the day and runs out early, that is the schedule rather than a fault.
6. Age. Find the label. Tanks typically last around eight to twelve years, and past that, repeated small faults are the tank telling you something.
What the symptom tells you
No hot water at all, suddenly. Electric: usually the upper element or the breaker. Gas: usually the pilot, thermocouple, or gas valve.
Warm rather than hot. Usually a failed lower element on an electric heater, or a thermostat that has drifted.
Hot water that runs out much faster than it used to. Sediment in the bottom of the tank taking up volume, or a failed lower element, or a dip tube.
Hot for a minute, then cold, then hot again. On a tankless unit that is the classic cold water sandwich, and it is normal behavior rather than a fault.
Hot water only at some faucets. A mixing valve or a cartridge in that fixture, not the heater.
Rumbling or popping from the tank. Sediment. Worth flushing, and on an old tank it is a sign of the end.
A smell of rotten eggs in the hot water only. Bacteria reacting with the anode rod. It is a known and fixable problem, and it is not the same as a gas smell, which is present in the air rather than in the water.
What is worth doing yourself
Relighting a pilot, following the printed instructions on the unit.
Resetting the high limit switch on an electric heater: a red button behind the upper access panel, with the breaker off before you open it. If it trips again, stop and call.
Flushing sediment, if the heater has never been done and is not ancient. Turn off the power or gas, connect a hose to the drain valve, and drain it to somewhere safe.
Checking the anode rod, which is the sacrificial part that keeps the tank from corroding. Replacing one at the right time genuinely extends the life of a tank.
What is not worth doing yourself: anything involving the gas valve, the burner assembly, or the wiring inside the unit.
When it is leaking rather than cold
Different problem, more urgent, and it has its own sequence. A leaking water heater, including why a tank leaking from the bottom cannot be repaired.
Repair or replace
Rough framework: an element, a thermostat, or a thermocouple is a small repair worth doing at any age. A gas valve or a major control on an old tank is close enough to replacement to think hard. A leaking tank is a replacement, always.
If the tank is past about ten years and needs anything substantial, replacing it on your schedule is better than replacing it on the day it fails and floods something. What that work costs.
The short version
Cold running normally means it is the heater, not the supply. Breaker or pilot, then the setting, then the age. Do not keep resetting a tripping breaker or a high limit switch. Do not raise the temperature past about 120 to compensate. A leaking tank is a replacement rather than a repair.