Draining your hot water heater annually is an important piece of routine maintenance you should perform on the unit to extend its lifespan and make sure it operates as efficiently as possible. Over time, sediment and rust can build up in the tank and draining the tank will get rid of this foreign material. If you've drained the tank and now are not getting any hot water, you should check a few things to see if you can't get the hot water flowing again, before calling in a service technician.
Check the cold water supply valve to see that you've reopened the cold water inlet leading into the hot water heater. If the valve has a round knob, it should be turned clockwise to open the valve. If the valve has a straight metal lever, the lever should be parallel to the inlet pipe for the valve to be open. If you turn on the hot water at a faucet and nothing comes out, this is an indication the supply valve was never reopened after draining the tank.
Remove the metal plate on the front of your hot water heater, if it's gas operated, and look through the glass panel to see if the pilot light is on. If it is, you'll see a small blue flame. If the pilot is off, reignite the pilot according to your unit's specific instructions, which will be printed on a label on the front of the hot water heater.
Look at the circuit breaker that provides power to the hot water heater, if it's an electric model, to see if it was tripped during the maintenance process. The inside of your electrical box should be numbered and labeled so you can easily identify which breaker is for the water heater. If the breaker is in the "Off" position, flip it on so the water heater will turn back on and start heating water.
Check the burner of a gas model by looking through the glass panel and see if the burner is on. If it is, you'll see a large blue flame. If the burner is not on, check the temperature setting on the unit, which can be adjusted by turning the large red temperature dial on the front. Raise the temperature setting until the burner ignites. To prevent burning someone with hot water, the temperature setting should not exceed 120 degrees Fahrenheit.
Wait for a period of time after refilling the water heater, because it's going to take time for the heating elements inside to actually heat the water. Don't expect to have hot water minutes after turning everything back on. The exact time it takes for a hot water heater to heat up a tank of water varies greatly from unit to unit. Water heaters come with different-sized burners, and the burner size differs even if the total water holding capacity is the same. The bigger the burner, the quicker results you will see.