Common water heater problems:
If you suddenly don’t have hot water, there are several things that could have happened.
If you have a gas-powered unit, check the gas line and pilot light. If the gas is on and you can’t get the pilot relit, you might just need to clean the gas burner or replace the thermocouple unit, which shuts off the gas if the pilot goes out.
On an electric unit, make sure the power is on, then reset the thermostat. If that doesn’t work, you’ll probably have to have the thermostat replaced.
If the tank itself is leaking, the unit is most likely defective and will have to be replaced immediately.
You should also check your pressure-relief valve, which guards the tank against rupture from steam pressure, on a yearly basis.